Recent interesting articles

Graphene1, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications12. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (~10–20 nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.

Z. Fei,

A. S. Rodin, W. Gannett, S. Dai, W. Regan, M. Wagner, M. K. Liu, A. S. McLeod, G. Dominguez, M. Thiemens, Antonio H. Castro Neto, F. Keilmann, A. Zettl, R. Hillenbrand, M. M. Fogler & D. N. Basov

[3]

Szept. 30. - Okt. 5. (2013)

Válogatta: Scherübl Zoltán

Simulations of electric-dipole spin resonance for spin-orbit coupled quantum dots in the Overhauser field: Fractional resonances and selection rules

E. N. Osika, B. Szafran, and M. P. Nowak

We consider spin rotations in single- and two-electron quantum dots that are driven by an external ac electric field with two mechanisms that couple the electron spatial motion and the spin degree of freedom: the spin-orbit interaction and a random fluctuation of the Overhauser field due to nuclear spin bath. We perform a systematic numerical simulation of the driven system using a finite-difference approach with an exact account taken for the electron-electron correlation. The simulation demonstrates that the electron oscillation in fluctuating nuclear field is translated into an effective magnetic field during the electron wave packet motion. The effective magnetic field drives the spin transitions according to the electric-dipole spin resonance mechanism. We find distinct signatures of selection rules for direct and higher order transitions in terms of the spin-orbital symmetries of the wave functions. The selection rules are violated by the random fluctuation of the Overhauser field.

http://prb.aps.org/abstract/PRB/v88/i16/e165302

Coherent quantum oscillations and echo measurements of a Si charge qubit

Zhan Shi, C. B. Simmons, Daniel R. Ward, J. R. Prance, R. T. Mohr, Teck Seng Koh, John King Gamble, Xian Wu, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, and M. A. Eriksson

Fast quantum oscillations of a charge qubit in a double quantum dot fabricated in a Si/SiGe heterostructure are demonstrated and characterized experimentally. The measured inhomogeneous dephasing time T∗ 2 ranges from 127 ps to 2.1 ns; it depends substantially on how the energy difference of the two qubit states varies with external voltages, consistent with a decoherence process that is dominated by detuning noise (charge noise that changes the asymmetry of the qubit’s double-well potential). In the regime with the shortest T∗ 2 , applying a charge-echo pulse sequence increases the measured inhomogeneous decoherence time from 127 to 760 ps, demonstrating that low-frequency noise processes are an important dephasing mechanism.

http://prb.aps.org/abstract/PRB/v88/i7/e075416

Spin-flip phonon-mediated charge relaxation in double quantum dots

J. Danon

We theoretically study the (1,1) triplet to (0,2) singlet relaxation rate in a lateral gate-defined double quantum dot tuned to the regime of Pauli spin blockade. We present a detailed derivation of the effective phonon density of states for this specific charge transition, keeping track of the contribution from piezoelectric as well as deformation potential electron-phonon coupling. We further investigate two different spin-mixing mechanisms which can couple the triplet and singlet states: a magnetic field gradient over the double dot (relevant at low external magnetic field) and spin-orbit interaction (relevant at high field), and we also indicate how the two processes could interfere at intermediate magnetic field. Finally, we show how to combine all results and evaluate the relaxation rate for realistic system parameters.

http://prb.aps.org/abstract/PRB/v88/i7/e075306

Quasibound states and evidence for a spin-1 Kondo effect in asymmetric quantum point contacts

Hao Zhang1, Phillip M. Wu2, and Albert M. Chang1

Linear conductance below 2e2/h shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero-bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even-odd effect, observable in the longer channels, is consistent with the formation of correlation-induced quasilocalized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin-1 Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple peak provides evidence suggestive of a spin singlet-triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons.

http://prb.aps.org/abstract/PRB/v88/i7/e075311

Weak localization and Raman study of anisotropically etched graphene antidots

Florian Oberhuber, Stefan Blien, Stefanie Heydrich, Fatemeh Yaghobian, Tobias Korn, Christian Schüller, Christoph Strunk, Dieter Weiss, and Jonathan Eroms

We study a crystallographic etching process of graphene nanostructures, where zigzag edges can be prepared selectively. The process involves heating exfoliated single-layer graphene samples with a predefined pattern of antidot arrays in an argon atmosphere at 820 ∘C, which selectively removes carbon atoms located on armchair sites. Atomic force microscopy and scanning electron microscopy cannot resolve the structure on the atomic scale. However, weak localization and Raman measurements, which both probe intervalley scattering at armchair edges, indicate that zigzag regions are enhanced compared to samples prepared with oxygen based reactive ion etching only.

http://apl.aip.org/resource/1/applab/v103/i14/p143111_s1

Direct Imaging of Atomic Scale Structure and Electronic Properties of GaAs Wurtzite and Zinc Blende Nanowire Surfaces

M. Hjort †, S. Lehmann †, J. Knutsson †, R. Timm †, D. Jacobsson †, E. Lundgren †, K.A. Dick †‡, and A. Mikkelsen

Using scanning tunneling microscopy and spectroscopy we study the atomic scale geometry and electronic structure of GaAs nanowires exhibiting controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal segments. We find that the nonpolar low-index surfaces {110}, {101̅0}, and {112̅0} are unreconstructed, unpinned, and without states in the band gap region. Direct comparison between Wz and Zb GaAs reveal a type-II band alignment and a Wz GaAs band gap of 1.52 eV

http://pubs.acs.org/doi/abs/10.1021/nl402424x

Ultraclean Single, Double, and Triple Carbon Nanotube Quantum Dots with Recessed Re Bottom Gates

Minkyung Jung *, Jens Schindele , Stefan Nau , Markus Weiss , Andreas Baumgartner , and Christian Schönenberger

We demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a carbon nanotube (CNT) by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in SiO2 by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional resist-based electron beam lithography. Unlike in previous work, the devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We specifically demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: (i) fully p-type with ballistic transport between the outermost contacts (over a length of 700 nm), (ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, (iii) n-type double QD, and (iv) triple bipolar QD where the middle QD has opposite doping (p-type). Our simple fabrication scheme opens up a route to more complex devices based on ultraclean CNTs, since it allows for postgrowth processing.

http://pubs.acs.org/doi/abs/10.1021/nl402455n

Direct Observation of Charge-Carrier Heating at WZ–ZB InP Nanowire Heterojunctions

Chaw Keong Yong †, Jennifer Wong-Leung ‡, Hannah J. Joyce †, James Lloyd-Hughes †, Qiang Gao ‡, H. Hoe Tan ‡, Chennupati Jagadish ‡, Michael B. Johnston †, and Laura M. Herz

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.

http://pubs.acs.org/doi/abs/10.1021/nl402050q#cor1

Magnetic polarons and large negative magnetoresistance in GaAs nanowires implanted with Mn ions

Sandeep Kumar , Waldomiro Paschoal Jr. , Andreas Johannes , Daniel Jacobsson , Christian Borschel , Anna Pertsova , chih-Han Wang , Maw-Kuen Wu , Carlo Canali , Carsten Ronning , Lars Samuelson , and Håkan Pettersson

We report on low-temperature magnetotransport and SQUID measurements on heavily doped Mn-implanted GaAs nanowires. SQUID data recorded at low magnetic fields exhibit clear signs of the onset of a spin-glass phase with a transition temperature of about 16 K. Magnetotransport experiments reveal a corresponding peak in resistance at 16 K and a large negative magnetoresistance, reaching 40 % at 1.6 K and 8 T. The negative magnetoresistance decreases at elevated temperatures and vanishes at about 100 K. We interpret our transport data in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn impurity spins, forming a paramagnetic/spin-glass phase.

http://pubs.acs.org/doi/abs/10.1021/nl402229r

p-Type InN Nanowires

S. Zhao †, B. H. Le †, D. P. Liu ‡, X. D. Liu ‡, M. G. Kibria †, T. Szkopek †, H. Guo ‡, and Z. Mi

In this Letter, we demonstrate that with the merit of nanowire structure and a self-catalytic growth process p-type InN can be realized for the first time by “direct” magnesium (Mg) doping. The presence of Mg acceptor energy levels in InN is confirmed by photoluminescence experiments, and a direct evidence of p-type conduction is demonstrated unambiguously by studying the transfer characteristics of InN nanowire field effect transistors. Moreover, the near-surface Fermi-level of InN can be tuned from nearly intrinsic to p-type degenerate by controlling Mg dopant incorporation, which is in contrast to the commonly observed electron accumulation on the grown surfaces of Mg-doped InN films. First-principle calculation using the VASP electronic package further shows that the p-type surface formed on Mg-doped InN nanowires is highly stable energetically.

http://pubs.acs.org/doi/abs/10.1021/nl4030819

Electrical Spin Injection and Detection in Mn5Ge3/Ge/Mn5Ge3 Nanowire Transistors

Jianshi Tang †, Chiu-Yen Wang †‡, Li-Te Chang †, Yabin Fan †, Tianxiao Nie †, Michael Chan †, Wanjun Jiang †, Yu-Ting Chen ‡, Hong-Jie Yang §, Hsing-Yu Tuan §, Lih-Juann Chen *‡, and Kang L. Wang

In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.

http://pubs.acs.org/doi/abs/10.1021/nl401238p

Szept. 26. - Okt. 1 (2013)

Válogatta: Magyarkuti András

Switching of ferroelectric polarization in epitaxial BaTiO3 films on silicon without a conducting bottom electrode

Catherine Dubourdieu, John Bruley, Thomas M. Arruda, Agham Posadas, Jean Jordan-Sweet, Martin M. Frank, Eduard Cartier, David J. Frank, Sergei V. Kalinin, Alexander A. Demkov & Vijay Narayanan

Epitaxial growth of SrTiO3 on silicon by molecular beam epitaxy has opened up the route to the integration of functional complex oxides on a silicon platform. Chief among them is ferroelectric functionality using perovskite oxides such as BaTiO3. However, it has remained a challenge to achieve ferroelectricity in epitaxial BaTiO3 films with a polarization pointing perpendicular to the silicon substrate without a conducting bottom electrode. Here, we demonstrate ferroelectricity in such stacks. Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal the presence of crystalline domains with the long axis of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and are reversibly switched with a phase change of 180°. Open, saturated hysteresis loops are recorded. Thus, ferroelectric switching of 8- to 40-nm-thick BaTiO3 films in metal–ferroelectric–semiconductor structures is realized, and field-effect devices using this epitaxial oxide stack can be envisaged.

Nature Nanotechnology (2013) doi:10.1038/nnano.2013.192

Interplay between Different Magnetisms in Cr-Doped Topological Insulators

Xufeng Kou, Murong Lang, Yabin Fan, Ying Jiang, Tianxiao Nie, Jianmin Zhang, Wanjun Jiang, Yong Wang, Yugui Yao, Liang He, and Kang L. Wang

Breaking the time-reversal-symmetry of topological insulators through magnetic doping has led to exotic physical discoveries. Here, we report the gate-dependent magneto-transport measurements on the Cr-doped (BixSb1–x)2Te3 thin films. With effective top-gate modulations, we demonstrate the presence of both the hole-mediated RKKY coupling and carrier-independent van Vleck magnetism in the magnetic TI systems. Most importantly, by varying the Cr doping concentrations from 2% to 20%, we unveil the interplay between the two magnetic orders and establish the valid approach to either enhance or suppress each individual contribution. The electric-field-controlled ferromagnetisms identified in the Cr-doped TI materials will serve as the fundamental step to further explore the TRS-breaking TI systems, and it may also help to expand the functionality of TI-based device for spintronics applications.

DOI: 10.1021/nn4038145

Charge Trapping Dynamics in PbS Colloidal Quantum Dot Photovoltaic Devices

Artem A. Bakulin, Stefanie Neutzner, Huib J. Bakker, Laurent Ottaviani, Damien Barakel, and Zhuoying Chen

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2–0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3–0.5 eV) is observed on the 0.1–10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

DOI: 10.1021/nn403190s

Introducing Carbon Diffusion Barriers for Uniform, High-Quality Graphene Growth from Solid Sources

Robert S. Weatherup, Carsten Baehtz, Bruno Dlubak, Bernhard C. Bayer, Piran R. Kidambi, Raoul Blume, Robert Schloegl, and Stephan Hofmann

Carbon diffusion barriers are introduced as a general and simple method to prevent premature carbon dissolution and thereby to significantly improve graphene formation from the catalytic transformation of solid carbon sources. A thin Al2O3 barrier inserted into an amorphous-C/Ni bilayer stack is demonstrated to enable growth of uniform monolayer graphene at 600 °C with domain sizes exceeding 50 μm, and an average Raman D/G ratio of <0.07. A detailed growth rationale is established via in situ measurements, relevant to solid-state growth of a wide range of layered materials, as well as layer-by-layer control in these systems.

DOI: 10.1021/nl401601x

Proximity Enhanced Quantum Spin Hall State in Graphene

Liangzhi Kou, Feiming Hu, Binghai Yan, Tim Wehling, Claudia Felser, Thomas Frauenheim, Changfeng Chen

Graphene is the first model system of two-dimensional topological insulator (TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in graphene, however, has eluded direct experimental detection because of its extremely small energy gap due to the weak spin-orbit coupling. Here we predict by ab initio calculations a giant (three orders of magnitude) proximity induced enhancement of the TI energy gap in the graphene layer that is sandwiched between thin slabs of Bi2Te3 (or Sb2Te3, MoTe2). This gap (~1 meV) is accessible by existing experimental techniques, and it can be further enhanced by tuning the interlayer distance via compression. We reveal by a tight-binding study that the QSH state in graphene is driven by the Kane-Mele interaction in competition with Kekule deformation and symmetry breaking. The present work identifies a new family of graphene-based TIs with an observable and controllable bulk energy gap in the graphene layer, thus opening a new avenue for direct verification and exploration of the long-sought QSH effect in graphene.

arXiv:1309.6653

Fractional quantum Hall states in charge-imbalanced bilayer systems

N. Thiebaut, N. Regnault, M.O. Goerbig

We study the fractional quantum Hall effect in a bilayer with charge-distribution imbalance induced, for instance, by a bias gate voltage. The bilayer can either be intrinsic or it can be formed spontaneously in wide quantum wells, due to the Coulomb repulsion between electrons. We focus on fractional quantum Hall effect in asymmetric bilayer systems at filling factor nu=4/11 and show that an asymmetric Halperin-like trial wavefunction gives a valid description of the ground state of the system.

arXiv:1309.7465

Emergent of Majorana Fermion mode and Dirac Equation in Cavity Quantum Electrodynamics

Sujit Sarkar

We present the results of low lying excitation of coupled optical cavity arrays. We derive the Dirac equation for this system and explain the existence of Majorana fermion mode in the system. We present quite a few analytical relations between the Rabi frequency oscillation and the atom-photon coupling strength to achieve the different physical situation of our study and also the condition for massless excitation in the system. We present several analytical relations between the Dirac spinor field, order and disorder operators for our systems. We also show that the Luttinger liquid physics is one of the intrinsic concept in our system.

arXiv:1309.7742

Júl. 01. - Szept. 25. (2013)

Válogatta: Balogh Zoltán

Electron heating in atomic-scale Au break junctions

Ruoyu Chen, Patrick J. Wheeler, M. Di Ventra, D. Natelson

Heating in nanoscale systems driven out of equilibrium is of fundamental importance, has ramifications for technological applications, and is a challenge to characterize experimentally. Prior experiments using nanoscale junctions have largely focused on heating of vibrational degrees of freedom, while heating of the electrons has been mostly neglected. We report measurements in atomic-scale Au break junctions, in which the bias-driven component of the current noise is used as a probe of the effective electronic temperature. At low biases ($<$ 150 mV) the noise is consistent with expectations of shot noise at a fixed electronic temperature. At higher biases, a nonlinear dependence of the noise power is observed consistent with a bias-driven increase in the effective electronic temperature, independent of the conductance of the junction. This result is inconsistent with a possible confounding effect of flicker noise. We discuss the implications of these observations for other nanoscale systems.

http://arxiv.org/abs/1306.6639

Mechanically controllable bi-stable states in a highly conductive single pyrazine molecular junction

Satoshi Kaneko, Carlo Motta, Gian Paolo Brivio and Manabu Kiguchi'

We report the fabrication of a highly conductive single pyrazine molecular junction with Pt leads. Mechanically controllable break-junction measurements at low temperatures show two distinct high and low conductance states. These conductance values are two orders of magnitude larger than those of a conventional single molecular junction with anchoring groups because of direct binding of the π conjugated molecule to a metal electrode with large density of states at the Fermi energy. Inelastic electron tunneling spectroscopy combined with density functional theory calculations highlights the vibration modes of the system for the two regimes. Theory allows us to assign the high and low conductance states of the molecular junction to two configurations in which the pyrazine axis is tilted and parallel with respect to the junction axis, respectively. Finally, we show that the pyrazine junction can be reversibly switched between the two bi-stable conductance states by mechanically stretching and relaxing the junction.

http://iopscience.iop.org/0957-4484/24/31/315201

Wide-Gap Semiconducting Graphene from Nitrogen-Seeded SiC

F. Wang, G. Liu, S. Rothwell, M. Nevius, A. Tejeda, A. Taleb-Ibrahimi, L. C. Feldman, P. I. Cohen, and E. H. Conrad

All carbon electronics based on graphene have been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of graphene electronics. We demonstrate a new approach to produce semiconducting graphene that uses a submonolayer concentration of nitrogen on SiC sufficient to pin epitaxial graphene to the SiC interface as it grows. The resulting buckled graphene opens a band gap greater than 0.7 eV in the otherwise continuous metallic graphene sheet.

http://pubs.acs.org/doi/abs/10.1021/nl402544n

Single-Molecule Conductance of Functionalized Oligoynes: Length Dependence and Junction Evolution

Pavel Moreno-García, Murat Gulcur, David Zsolt Manrique, Thomas Pope, Wenjing Hong, Veerabhadrarao Kaliginedi, Cancan Huang, Andrei S. Batsanov, Martin R. Bryce, Colin Lambert, and Thomas Wandlowski

We report a combined experimental and theoretical investigation of the length dependence and anchor group dependence of the electrical conductance of a series of oligoyne molecular wires in single-molecule junctions with gold contacts. Experimentally, we focus on the synthesis and properties of diaryloligoynes with n = 1, 2, and 4 triple bonds and the anchor dihydrobenzo[b]thiophene (BT). For comparison, we also explored the aurophilic anchor group cyano (CN), amino (NH2), thiol (SH), and 4-pyridyl (PY). Scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques are employed to investigate single-molecule conductance characteristics. The BT moiety is superior as compared to traditional anchoring groups investigated so far. BT-terminated oligoynes display a 100% probability of junction formation and possess conductance values which are the highest of the oligoynes studied and, moreover, are higher than other conjugated molecular wires of similar length. Density functional theory (DFT)-based calculations are reported for oligoynes with n = 1–4 triple bonds. Complete conductance traces and conductance distributions are computed for each family of molecules. The sliding of the anchor groups leads to oscillations in both the electrical conductance and the binding energies of the studied molecular wires. In agreement with experimental results, BT-terminated oligoynes are predicted to have a high electrical conductance. The experimental attenuation constants βH range between 1.7 nm–1 (CN) and 3.2 nm–1 (SH) and show the following trend: βH(CN) < βH(NH2) < βH(BT) < βH(PY) ≈ βH(SH). DFT-based calculations yield lower values, which range between 0.4 nm–1 (CN) and 2.2 nm–1 (PY).

http://pubs.acs.org/doi/abs/10.1021/ja4015293

Single Molecule Conductance, Thermopower, and Transition Voltage

Shaoyin Guo, Gang Zhou, and Nongjian Tao

We have measured the thermopower as well as other important charge transport quantities, including conductance, current–voltage characteristics, and transition voltage of single molecules. The thermopower has little correlation with the conductance, but it decreases with the transition voltage, which is consistent with a theory based on Landauer’s formula. Since the transition voltage reflects the molecular energy level alignment, our finding also shows that the thermopower provides valuable information about the relative alignment between the molecular energy levels and the electrodes’ Fermi energy level.

http://pubs.acs.org/doi/abs/10.1021/nl4021073

Turning off Hydrogen To Realize Seeded Growth of Subcentimeter Single-Crystal Graphene Grains on Copper

Lin Gan and Zhengtang Luo

Subcentimeter single-crystalline graphene grains, with diameter up to 5.9 mm, have been successfully synthesized by tuning the nucleation density during atmospheric pressure chemical vapor deposition. Morphology studies show the existence of a single large nanoparticle (>20 nm in diameter) at the geometric center of those graphene grains. Similar size particles were produced by slightly oxidizing the copper surface to obtain oxide nanoparticles in Ar-only environments, followed by reduction into large copper nanoparticles under H2/Ar environment, and are thus explained to be the main constituent nuclei for graphene growth. On this basis, we were able to control the nanoparticle density by adjusting the degree of oxidation and hydrogen annealing duration, thereby controlling nucleation density and consequently controlling graphene grain sizes. In addition, we found that hydrogen plays dual roles on copper morphology during the whole growth process, that is, removing surface irregularities and, at the same time, etching the copper surface to produce small nanoparticles that have only limited effect on nucleation for graphene growth. Our reported approach provides a highly efficient method for production of graphene film with long-range electronic connectivity and structure coherence.

http://pubs.acs.org/doi/abs/10.1021/nn404393b

A current-driven single-atom memory

C. Schirm, M. Matt, F. Pauly, J. C. Cuevas, P. Nielaba & E. Scheer

The possibility of fabricating electronic devices with functional building blocks of atomic size is a major driving force of nanotechnology1. The key elements in electronic circuits are switches, usually realized by transistors, which can be configured to perform memory operations. Electronic switches have been miniaturized all the way down to the atomic scale2, 3, 4, 5, 6, 7, 8, 9. However, at such scales, three-terminal devices are technically challenging to implement. Here we show that a metallic atomic-scale contact can be operated as a reliable and fatigue-resistant two-terminal switch. We apply a careful electromigration protocol to toggle the conductance of an aluminium atomic contact between two well-defined values in the range of a few conductance quanta. Using the nonlinearities of the current–voltage characteristics caused by superconductivity10 in combination with molecular dynamics and quantum transport calculations, we provide evidence that the switching process is caused by the reversible rearrangement of single atoms. Owing to its hysteretic behaviour with two distinct states, this two-terminal switch can be used as a non-volatile information storage element.

http://www.nature.com/nnano/journal/v8/n9/full/nnano.2013.170.html

Impact of Molecular Symmetry on Single-Molecule Conductance

Emma J. Dell, Brian Capozzi, Kateri H. DuBay, Timothy C. Berkelbach, Jose Ricardo Moreno, David R. Reichman, Latha Venkataraman, and Luis M. Campos

We have measured the single-molecule conductance of a family of bithiophene derivatives terminated with methyl sulfide gold-binding linkers using a scanning tunneling microscope based break-junction technique. We find a broad distribution in the single-molecule conductance of bithiophene compared with that of a methyl sulfide terminated biphenyl. Using a combination of experiments and calculations, we show that this increased breadth in the conductance distribution is explained by the difference in 5-fold symmetry of thiophene rings as compared to the 6-fold symmetry of benzene rings. The reduced symmetry of thiophene rings results in a restriction on the torsion angle space available to these molecules when bound between two metal electrodes in a junction, causing each molecular junction to sample a different set of conformers in the conductance measurements. In contrast, the rotations of biphenyl are essentially unimpeded by junction binding, allowing each molecular junction to sample similar conformers. This work demonstrates that the conductance of bithiophene displays a strong dependence on the conformational fluctuations accessible within a given junction configuration, and that the symmetry of such small molecules can significantly influence their conductance behaviors.

http://pubs.acs.org/doi/abs/10.1021/ja4055367

Experimental determination of conduction channels in atomic scale conductors based on shot noise measurements

Ran Vardimon, Marina Klionsky, Oren Tal

We present an experimental procedure for obtaining the conduction channels of low-dimensional conductors based on shot noise measurements. The transmission coefficient for each channel is determined numerically from the measured conductance and Fano factor. The channel analysis is demonstrated for atomic contacts of Ag, Au, Al and Pt, showing their channel evolution as a function of conductance and mechanical elongation. This approach can be readily applied to map the conduction channels in a wide range of nanoscale conductors under different conditions.

http://arxiv.org/abs/1308.3425

Ápr. 28. - Máj. 5. (2013)

Válogatta: Balla Péter

Observation of non-sinusoidal current-phase relation in graphene Josephson junctions

Christopher English, David Hamilton, Cesar Chialvo, Ion Moraru, Nadya Mason, Dale Van Harlingen

We present direct measurements of the current-phase relation for lateral Josephson junctions with a graphene barrier, obtained by a phase-sensitive SQUID interferometry technique. We find that the current-phase relation is forward skewed with respect to the commonly observed sinusoidal behavior for short junctions in the quasi-ballistic transport regime, consistent with predictions for the behavior of Dirac fermions in a Josephson junction. The skewness increases with critical current and decreases sharply with increasing temperature.

http://xxx.lanl.gov/abs/1305.0327

Group theoretical and topological analysis of the quantum spin Hall effect in silicene

Florian Geisser, Jan Carl Budich, Björn Trauzettel

Silicene consists of a monolayer of silicon atoms in a buckled honeycomb structure. It was recently discovered that the symmetry of such a system allows for interesting Rashba spin-orbit effects. A perpendicular electric field is able to couple to the sublattice pseudospin, making it possible to electrically tune and close the band gap. Therefore, external electric fields may generate a topological phase transition from a topological insulator to a normal insulator (or semimetal) and vice versa. The contribution of the present article to the study of silicene is twofold: First, we perform a group theoretical analysis to systematically construct the Hamiltonian in the vicinity of the $K$ points of the Brillouin zone and discover a new, but symmetry allowed term. Subsequently, we identify a tight binding model that corresponds to the group theoretically derived Hamiltonian near the $K$ points. Second, we start from this tight binding model to analyze the topological phase diagram of silicene by an explicit calculation of the $Z_2$ topological invariant of the band structure. To this end, we calculate the $Z_2$ topological invariant of the honeycomb lattice in a manifestly gauge invariant way which allows us to include $S_z$ symmetry breaking terms -- like Rashba spin orbit interaction -- into the topological analysis. Interestingly, we find that the interplay of two Rashba terms can generate a non-trivial quantum spin Hall phase in silicene. This is in sharp contrast to the more extensively studied honeycomb system graphene where Rashba spin orbit interaction is known to compete with the quantum spin Hall effect in a potentially detrimental way.

http://xxx.lanl.gov/abs/1305.0766

Artificial spin ice: The heat is on

Christopher Marrows

Model magnetic systems known as artificial spin ices have almost always been found in frozen, athermal states. But an artificial spin ice that is designed to be thermally active has now been imaged as it explores its frustrated energy landscape.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2638.html

Controllable optical negative refraction and phase conjugation in graphite thin films

Hayk Harutyunyan, Ryan Beams, Lukas Novotny

Optical metamaterials have demonstrated remarkable physical properties, including cloaking, optical magnetism and negative refraction1, 2, 3. The last of these has attracted particular interest, mainly because of its promise for super-resolution imaging4, 5, 6. However, the widespread use of negative refraction at optical frequencies is challenged by high losses and strong dispersion effects, which typically limit operation to narrow frequency bands7. Here we use degenerate four-wave mixing to demonstrate controllable negative refraction at a graphite thin film, which acts as a highly efficient phase-conjugating surface. The scheme has very low loss because of the negligible thickness of the nonlinear material and it ensures broadband operation due to the linear band structure of graphene.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2618.html

Direct observation and dynamics of spontaneous skyrmion-like magnetic domains in a ferromagnet

Masahiro Nagao, Yeong-Gi So, Hiroyuki Yoshida, Masaaki Isobe, Toru Hara, Kazuo Ishizuka, Koji Kimoto

The structure and dynamics of submicrometre magnetic domains are the main factors determining the physical properties of magnetic materials1, 2. Here, we report the first observation of skyrmion-like magnetic nanodomains in a ferromagnetic manganite, La0.5Ba0.5MnO3, using Lorentz transmission electron microscopy (LTEM). The skyrmion-like magnetic domains appear as clusters above the Curie temperature. We found that the repeated reversal of magnetic chirality is caused by thermal fluctuation. The closely spaced clusters exhibit dynamic coupling, and the repeated magnetization reversal becomes fully synchronized with the same chirality. Quantitative analysis of such dynamics was performed by LTEM to directly determine the barrier energy for the magnetization reversal of skyrmion-like nanometre domains. This study is expected to pave the way for further investigation of the unresolved nature and dynamics of magnetic vortex-like nanodomains.

http://www.nature.com/nnano/journal/v8/n5/full/nnano.2013.69.html

DNA sequencing using electrical conductance measurements of a DNA polymerase

Yu-Shiun Chen, Chia-Hui Lee, Meng-Yen Hung, Hsu-An Pan, Jin-Chern Chiou, G. Steven Huang

The development of personalized medicine—in which medical treatment is customized to an individual on the basis of genetic information—requires techniques that can sequence DNA quickly and cheaply. Single-molecule sequencing technologies, such as nanopores, can potentially be used to sequence long strands of DNA without labels or amplification, but a viable technique has yet to be established. Here, we show that single DNA molecules can be sequenced by monitoring the electrical conductance of a phi29 DNA polymerase as it incorporates unlabelled nucleotides into a template strand of DNA. The conductance of the polymerase is measured by attaching it to a protein transistor that consists of an antibody molecule (immunoglobulin G) bound to two gold nanoparticles, which are in turn connected to source and drain electrodes. The electrical conductance of the DNA polymerase exhibits well-separated plateaux that are ~3 pA in height. Each plateau corresponds to an individual base and is formed at a rate of ~22 nucleotides per second. Additional spikes appear on top of the plateaux and can be used to discriminate between the four different nucleotides. We also show that the sequencing platform works with a variety of DNA polymerases and can sequence difficult templates such as homopolymers.

http://www.nature.com/nnano/journal/vaop/ncurrent/nnano.2013.71/metrics

Immobilizing Individual Atoms beneath a Corrugated Single Layer of Boron Nitride

Huanyao Cun, Marcella Iannuzzi, Adrian Hemmi, Silvan Roth, Jürg Osterwalder, and Thomas Greber

Single atoms, and in particular the least reactive noble gases, are difficult to immobilize at room temperature. Ion implantation into a crystal lattice has this capability, but the randomness of the involved processes does not permit much control over their distribution within the solid. Here we demonstrate that the boron nitride nanomesh, a corrugated single layer of hexagonal boron nitride (h-BN) with a 3.2 nm honeycomb superstructure formed on a Rh(111) surface, can trap individual argon atoms at distinct subsurface sites at room temperature. A kinetic energy window for implantation is identified where the argon ions can penetrate the h-BN layer but not enter the Rh lattice. Scanning tunneling microscopy and photoemission data show the presence of argon atoms at two distinct sites within the nanomesh unit cell, confirmed also by density functional theory calculations. The single atom implants are stable in air. Annealing of implanted structures to 900 K induces the formation of highly regular holes of 2 nm diameter in the h-BN layer with adjacent flakes of the same size found on top of the layer. We explain this “can-opener” effect by the presence of a vacancy defect, generated during the penetration of the Ar ion through the h-BN lattice, and propagating along the rim of a nanomesh pore where the h-BN lattice is highly bent. The reported effects are also observed in graphene on ruthenium and for neon atoms.

http://pubs.acs.org/doi/abs/10.1021/nl400449y

In vivo magnetic resonance imaging of hyperpolarized silicon particles

M. C. Cassidy, H. R. Chan, B. D. Ross, P. K. Bhattacharya, C. M. Marcus

Silicon-based micro- and nanoparticles have gained popularity in a wide range of biomedical applications due to their biocompatibility and biodegradability in vivo, as well as their flexible surface chemistry, which allows drug loading, functionalization and targeting. Here, we report direct in vivo imaging of hyperpolarized 29Si nuclei in silicon particles by magnetic resonance imaging. Natural physical properties of silicon provide surface electronic states for dynamic nuclear polarization, extremely long depolarization times, insensitivity to the in vivo environment or particle tumbling, and surfaces favourable for functionalization. Potential applications to gastrointestinal, intravascular and tumour perfusion imaging at subpicomolar concentrations are presented. These results demonstrate a new background-free imaging modality applicable to a range of inexpensive, readily available and biocompatible silicon particles.

http://www.nature.com/nnano/journal/v8/n5/abs/nnano.2013.65.html

Long-distance coherent coupling in a quantum dot array

F. R. Braakman, P. Barthelemy, C. Reichl, W. Wegscheider, L. M. K. Vandersypen

Controlling long-distance quantum correlations is central to quantum computation and simulation. In quantum dot arrays, experiments so far rely on nearest-neighbour couplings only, and inducing long-distance correlations requires sequential local operations. Here, we show that two distant sites can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau–Zener–Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantum dot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.67.html

Nanotechnology: The can-opener effect

Bart Verberck

Noble gas atoms are hard to tame. They seldom participate in chemical reactions, and only condense into liquids and solids at temperatures far below 0 °C. Yet Huanyao Cun and colleagues now show that argon atoms can be coaxed into regular two-dimensional lattices at room temperature.

http://www.nature.com/nphys/journal/v9/n5/full/nphys2623.html

Optical Far-Field Method with Subwavelength Accuracy for the Determination of Nanostructure Dimensions in Large-Area Samples

Nicklas Anttu, Magnus Heurlin, Magnus T. Borgström, Mats-Erik Pistol, H. Q. Xu, Lars Samuelson

The physical, chemical, and biological properties of nanostructures depend strongly on their geometrical dimensions. Here we present a fast, noninvasive, simple-to-perform, purely optical method that is capable of characterizing nanostructure dimensions over large areas with an accuracy comparable to that of scanning electron microscopy. This far-field method is based on the analysis of unique fingerprints in experimentally measured reflectance spectra using full three-dimensional optical modeling. We demonstrate the strength of our method on large-area (millimeter-sized) arrays of vertical InP nanowires, for which we simultaneously determine the diameter and length as well as cross-sample morphological variations thereof. Explicitly, the diameter is determined with an accuracy better than 10 nm and the length with an accuracy better than 30 nm. The method is versatile and robust, and we believe that it will provide a powerful and standardized measurement technique for large-area nanostructure arrays suitable for both research and industrial applications.

Understanding Self-Aligned Planar Growth of InAs Nanowires

Yunlong Zi, Kyooho Jung, Dmitri Zakharov, Chen Yang

Semiconducting nanowires have attracted lots of attention because of their potential applications. Compared with free-standing nanowires, self-aligned planar nanowires grown epitaxially on the substrate have shown advantageous properties such as being twin defect free and ready for device fabrication, opening potentials for the large-scale device applications. Understanding of planar nanowire growth, which is essential for selective growth of planar vs free-standing wires, is still limited. In this paper, we reported different growth behaviors for self-aligned planar and free-standing InAs nanowires under identical growth conditions. We present a new model based on a revised Gibbs–Thomson equation for the planar nanowires. Using this model, we predicted and successfully confirmed through experiments that higher arsenic vapor partial pressure promoted free-standing InAs nanowire growth. A smaller critical diameter for planar nanowire growth was predicted and achieved experimentally. Successful control and understanding of planar and free-standing nanowire growth established in our work opens up the potential of large-scale integration of self-aligned nanowires for practical device applications.

http://pubs.acs.org/doi/abs/10.1021/nl4010332

Ápr. 20. - Ápr. 27. (2013)

Válogatta: Márton Attila

A ballistic pn junction in suspended graphene with split bottom gates

Anya L. Grushina, Dong-Keun Ki, Alberto F. Morpurgo

We have developed a process to fabricate suspended graphene devices with local bottom gates, and tested it by realizing electrostatically controlled pn junctions on a suspended graphene mono-layer nearly 2 micrometers long. Measurements as a function of gate voltage, magnetic field, bias, and temperature exhibit characteristic Fabry-Perot oscillations in the cavities formed by the pn junction and each of the contacts, with transport occurring in the ballistic regime. Our results demonstrate the possibility to achieve a high degree of control on the local electronic properties of ultra-clean suspended graphene layers, a key aspect for the realization of new graphene nanostructures.

http://xxx.lanl.gov/abs/1304.6844

Ballistic interferences in suspended graphene

Peter Rickhaus, Romain Maurand, � Ming-Hao Liu, Markus Weiss, Klaus Richter, and Christian Schonenberger

Graphene is a 2-dimensional (2D) carbon allotrope with the atoms arranged in a honeycomb lattice [1]. The low-energy electronic excitations in this 2D crystal are described by massless Dirac fermions that have a linear dispersion relation similar to photons [2, 3]. Taking advantage of this \optics-like" electron dynamics, generic optical elements like lenses, beam splitters and wave guides have been proposed for electrons in engineered ballistic graphene [4, 5]. Tuning of these elements relies on the ability to adjust the carrier concentration in defi�ned areas, including the possibility to create bipolar regions of opposite charge (p-n regions). However, the combination of ballistic transport and complex electrostatic gating remains challenging. Here, we report on the fabrication and characterization of fully suspended graphene p-n junctions. By local electrostatic gating, resonant cavities can be de�ned, leading to complex Fabry-P�erot interference patterns in the unipolar and the bipolar regime. The amplitude of the observed conductance oscillations accounts for quantum interference of electrons that propagate ballistically over long distances exceeding 1 �m. We also demonstrate that the visibility of the interference pattern is enhanced by Klein collimation at the p-n interface [6, 7]. This �nding paves the way to more complex gate-controlled ballistic graphene devices and brings electron optics in graphene closer to reality.

http://arxiv.org/pdf/1304.6590v1.pdf

(courtesy of Makk P.)

Competition between the Superconducting Proximity Effect and Coulomb Interactions in a Graphene Andreev Interferometer

Fabio Deon, Sandra Šopić, Alberto F. Morpurgo

We have investigated transport through graphene Andreev interferometers exhibiting reentrance of the superconducting proximity effect. We observed a crossover in the Andreev conductance oscillations as a function of gate voltage ($V_{BG}$). At high $V_{BG}$ the energy-dependent oscillation amplitude exhibits a scaling predicted for non-interacting electrons, which breaks down at low $V_{BG}$. The phenomenon is a manifestation of electron-electron interactions, whose main effect is to shorten the single-particle phase coherence time $\tau_\phi$. These results indicate that graphene provides a useful experimental platform to investigate the competition between superconducting proximity effect and interactions.

http://xxx.lanl.gov/abs/1304.6578

Topological Insulator Materials

Yoichi Ando

Topological insulators represent a new quantum state of matter which is characterized by peculiar edge or surface states that show up due to a topological character of the bulk wave functions. This review presents a pedagogical account on topological insulator materials with an emphasis on basic theory and materials properties. After presenting a historical perspective and basic theories of topological insulators, it discusses all the topological insulator materials discovered to date, with some illustrative descriptions of the developments in materials discoveries in which the author was involved. A summary is given for possible ways to confirm the topological nature in a candidate material. Various synthesis techniques as well as the defect chemistry that became important for realizing bulk-insulating samples are discussed. Characteristic properties of topological insulators are discussed with an emphasis on transport properties. In particular, the Dirac fermion physics and the resulting peculiar quantum oscillation patterns are discussed in detail. It is emphasized that proper analyses of quantum oscillations make it possible to unambiguously identify surface Dirac fermions through transport measurements. The prospects of topological insulator materials for elucidating novel quantum phenomena that await discovery conclude the review.

http://xxx.lanl.gov/abs/1304.5693

Charge-Carrier Screening in Single-Layer Graphene

David A. Siegel, William Regan, Alexei V. Fedorov, A. Zettl, Alessandra Lanzara

The effect of charge-carrier screening on the transport properties of a neutral graphene sheet is studied by directly probing its electronic structure. We find that the Fermi velocity, Dirac point velocity, and overall distortion of the Dirac cone are renormalized due to the screening of the electron-electron interaction in an unusual way. We also observe an increase of the electron mean free path due to the screening of charged impurities. These observations help us to understand the basis for the transport properties of graphene, as well as the fundamental physics of these interesting electron-electron interactions at the Dirac point crossing.

http://xxx.lanl.gov/abs/1304.5693

Enhanced Optical Second-Harmonic Generation from the Current-Biased Graphene/SiO2/Si(001) Structure

Yong Q. An , Florence Nelson , Ji Ung Lee , and Alain C. Diebold

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct current electric current in graphene. Measurements of rotational-anisotropy SHG revealed that the current-induced SHG from the current-biased graphene/SiO2/Si(001) structure undergoes a phase inversion as the measurement location on graphene is shifted laterally along the current flow direction. The enhancement is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The phase inversion is due to the positive-to-negative polarity switch in the current direction of the trapped charges at the current-biased graphene/SiO2 interface.

http://xxx.lanl.gov/abs/1304.5837

Electrical transport in C-doped GaAs nanowires: surface effects

Alberto Casadei, Jil Schwender, Eleonora Russo-Averchi, Daniel Rüffer, Martin Heiss, Esther Alarcó-Lladó, Fauzia Jabeen, Mohammad Ramezani, Kornelius Nielsch, Anna Fontcuberta I Morral

The resistivity and the mobility of Carbon doped GaAs nanowires have been studied for different doping concentrations. Surface effects have been evaluated by comparing upassivated with passivated nanowires. We directly see the influence of the surface: the pinning of the Fermi level and the consequent existence of a depletion region lead to an increase of the mobility up to 30 cm^2/(V*s) for doping concentrations lower than 3*10^18 cm^-3. Electron beam induced current measurements show that the minority carrier diffusion path can be as high as 190 nm for passivated nanowires.

http://xxx.lanl.gov/abs/1304.5891

One-dimensional transport of interacting particles: Currents, density profiles, phase diagrams and symmetries

Marcel Dierl, Mario Einax, Philipp Maass

Driven lattice gases serve as canonical models for investigating collective transport phenomena and properties of non-equilibrium steady states (NESS). Here we study one-dimensional transport with nearest-neighbor interactions both in closed bulk systems and in open channels coupled to two particle reservoirs at the ends of the channel. For the widely employed Glauber rates we derive an exact current-density relation in the bulk for unidirectional hopping. An approach based on time-dependent density functional theory provides a good description of the kinetics. For open systems, the system-reservoir couplings are shown to have a striking influence on boundary-induced phase diagrams. The role of particle-hole symmetry is discussed and its consequence on the topology of the phase diagrams. It is furthermore demonstrated that systems with weak bias can be mapped onto systems with unidirectional hopping.

http://xxx.lanl.gov/abs/1304.5953

Six-electron semiconductor double quantum dot qubits

Erik Nielsen, Edwin Barnes, J. P. Kestner, S. Das Sarma

We consider a double-quantum-dot (DQD) qubit which contains six electrons instead of the usual one or two. In this spin qubit, quantum information is encoded in a low-lying singlet-triplet space much as in the case of a two-electron DQD qubit. We find that initialization, manipulation, and read- out can be performed similarly to the two-electron case, and that energy gaps remain large enough that these operations can be performed robustly. We consider DQD potentials with parameters chosen to be representative of current experimental capabilities. Results are obtained using two complementary full configuration interaction methods.

http://xxx.lanl.gov/abs/1304.6064

Dynamic and Electronic Transport Properties of DNA Translocation through Graphene Nanopores

Stanislav M. Avdoshenko , Daijiro Nozaki , Claudia Gomes da Rocha , Jhon W. González , Myeong H. Lee , Rafael Gutierrez , and Gianaurelio Cuniberti

Graphene layers have been targeted in the last years as excellent host materials for sensing a remarkable variety of gases and molecules. Such sensing abilities can also benefit other important scientific fields such as medicine and biology. This has automatically led scientists to probe graphene as a potential platform for sequencing DNA strands. In this work, we use robust numerical tools to model the dynamic and electronic properties of molecular sensor devices composed of a graphene nanopore through which DNA molecules are driven by external electric fields. We performed molecular dynamic simulations to determine the relation between the intensity of the electric field and the translocation time spent by the DNA to pass through the pore. Our results reveal that one can have extra control on the DNA passage when four additional graphene layers are deposited on the top of the main graphene platform containing the pore in a 2 × 2 grid arrangement. In addition to the dynamic analysis, we carried electronic transport calculations on realistic pore structures with diameters reaching nanometer scales. The transmission obtained along the graphene sensor at the Fermi level is affected by the presence of the DNA. However, it is rather hard to distinguish the respective nucleobases. This scenario can be significantly altered when the transport is conducted away from the Fermi level of the graphene platform. Under an energy shift, we observed that the graphene pore manifests selectiveness toward DNA nucleobases.

http://pubs.acs.org/doi/abs/10.1021/nl304735k

The Role of External Defects in Chemical Sensing of Graphene Field-Effect Transistors

B. Kumar , K. Min , M. Bashirzadeh , A. Barati Farimani , M.-H. Bae , D. Estrada , Y. D. Kim , P. Yasaei , Y. D. Park , E. Pop , N. R. Aluru , and A. Salehi-Khojin

A fundamental understanding of chemical sensing mechanisms in graphene-based chemical field-effect transistors (chemFETs) is essential for the development of next generation chemical sensors. Here we explore the hidden sensing modalities responsible for tailoring the gas detection ability of pristine graphene sensors by exposing graphene chemFETs to electron donor and acceptor trace gas vapors. We uncover that the sensitivity (in terms of modulation in electrical conductivity) of pristine graphene chemFETs is not necessarily intrinsic to graphene, but rather it is facilitated by external defects in the insulating substrate, which can modulate the electronic properties of graphene. We disclose a mixing effect caused by partial overlap of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of adsorbed gas molecules to explain graphene’s ability to detect adsorbed molecules. Our results open a new design space, suggesting that control of external defects in supporting substrates can lead to tunable graphene chemical sensors, which could be developed without compromising the intrinsic electrical and structural properties of graphene.

http://pubs.acs.org/doi/abs/10.1021/nl304734g

Direct Observation of Hole Transfer from Semiconducting Polymer to Carbon Nanotubes

Fei Lan and Guangyong Li

Carbon nanotubes have been proven to play significant roles in polymer-based solar cells. However, there is intensive debate on whether carbon nanotube behaves as a donor or acceptor in the semiconducting polymer:carbon nanotube composite. In this paper, we report a direct observation via Kelvin probe force microscopy (KPFM) that single walled carbon nanotubes (SWNTs) behave as hole transporting channels in poly(3-hexylthiophene-2,5-diyl) (P3HT)/SWNT heterojunctions. By comparing the surface potential (SP) change of SWNT in dark and under illumination, we observed that electrons are blocked from SWNT while holes are transferred to SWNT. This observation can be well-explained by our proposed band alignment model of P3HT/SWNT heterojunction. The finding is further verified by hole mobility measurement using the space charge limited current (SCLC) method. SCLC results indicate that the existence of small amount of SWNT (wt 0.5%) promotes device hole mobility to around 15-fold, indicating SWNT act as hole transfer channel. Our finding of hole transporting behavior of SWNT in P3HT/SWNT blend will provide a useful guidance for enhancing the performance of polymer solar cells by carbon nanotubes.

http://pubs.acs.org/doi/abs/10.1021/nl400395c

A nanoscale combing technique for the large-scale assembly of highly aligned nanowires

Jun Yao, Hao Yan & Charles M. Lieber

The controlled assembly of nanowires is a key challenge in the development of a range of bottom-up devices1, 2. Recent advances2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 in the post-growth assembly of nanowires and carbon nanotubes have led to alignment ratios of 80–95% for a misalignment angle of ±5° (refs 5, 12, 13, 14) and allowed various multiwire devices to be fabricated6, 10, 11, 12, 13, 19. However, these methods still create a significant number of crossing defects, which restricts the development of device arrays and circuits based on single nanowires/nanotubes. Here, we show that a nanocombing assembly technique, in which nanowires are anchored to defined areas of a surface and then drawn out over chemically distinct regions of the surface, can yield arrays with greater than 98.5% of the nanowires aligned to within ±1° of the combing direction. The arrays have a crossing defect density of ~0.04 nanowires per µm and efficient end registration at the anchoring/combing interface. With this technique, arrays of single-nanowire devices are tiled over chips and shown to have reproducible electronic properties. We also show that nanocombing can be used for laterally deterministic assembly, to align ultralong (millimetre-scale) nanowires to within ±1° and to assemble suspended and crossed nanowire arrays.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.55.html

Dynamic switching of the spin circulation in tapered magnetic nanodisks

V. Uhlíř, M. Urbánek, L. Hladík, J. Spousta, M-Y. Im, P. Fischer, N. Eibagi, J. J. Kan, E. E. Fullerton & T. Šikola

Magnetic vortices are characterized by the sense of in-plane magnetization circulation and by the polarity of the vortex core. With each having two possible states, there are four possible stable magnetization configurations that can be utilized for a multibit memory cell. Dynamic control of vortex core polarity has been demonstrated using both alternating and pulsed magnetic fields and currents. Here, we show controlled dynamic switching of spin circulation in vortices using nanosecond field pulses by imaging the process with full-field soft X-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core, and the reversal is achieved at significantly reduced field amplitudes when compared with static switching. We further show that both the field pulse amplitude and duration required for efficient circulation reversal can be controlled by appropriate selection of the disk geometry.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.66.html

Quantum physics: A grip on misbehaviour

Stefano Pironio & Dorit Aharonov

Physicists have come up with a way to characterize and command untrusted quantum systems. Two experts discuss the significance of these findings for fundamental science and for practical quantum computation and cryptography.

http://www.nature.com/nature/journal/v496/n7446/full/496436a.html

Diamond shows promise for a quantum Internet

Richard Van Noorden

Today's Internet runs on linked silicon chips, but a future quantum version might be built from diamond crystals. Physicists report today in Nature1 that they have entangled information kept in pieces of diamond 3 metres apart, so that measuring the state of one quantum bit (qubit) instantly fixes the state of the other - a step necessary for exchanging quantum information over large distances.

http://www.nature.com/news/diamond-shows-promise-for-a-quantum-internet-1.12870

Ápr. 6. - Ápr. 19. (2013)

Válogatta: Fülöp Bálint

Electrically tunable three-dimensional g-factor anisotropy in single InAs self-assembled quantum dots

Three-dimensional anisotropy of the Landé g factor and its electrical modulation are studied for single uncapped InAs self-assembled quantum dots (QDs). The g factor is evaluated from measurement of inelastic cotunneling via Zeeman substates in the QD for various magnetic field directions. We find that the value and anisotropy of the g factor depends on the type of orbital state which arises from the three-dimensional confinement anisotropy of the QD potential. Furthermore, the g factor and its anisotropy are electrically tuned by a side gate which modulates the confining potential.

http://prb.aps.org/abstract/PRB/v87/i16/e161302

Enhanced Optical Second-Harmonic Generation from the Current-Biased Graphene/SiO2/Si(001) Structure

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct current electric current in graphene. Measurements of rotational-anisotropy SHG revealed that the current-induced SHG from the current-biased graphene/SiO2/Si(001) structure undergoes a phase inversion as the measurement location on graphene is shifted laterally along the current flow direction. The enhancement is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The phase inversion is due to the positive-to-negative polarity switch in the current direction of the trapped charges at the current-biased graphene/SiO2 interface.

http://pubs.acs.org/doi/abs/10.1021/nl4004514

Excitation-induced dephasing in a resonantly driven InAs/GaAs quantum dot

We report on coherent emission of the neutral exciton state in a single semiconductor self-assembled InAs/GaAs quantum dot embedded in a one-dimensional waveguide, under resonant picosecond pulsed excita- tion. Direct measurements of the radiative lifetime and coherence time are performed as a function of excitation power and temperature. The characteristic damping of Rabi oscillations which is observed, is attributed to an excitation-induced dephasing due to a resonant coupling between the emitter and the acoustic phonon bath of the matrix. Other sources responsible for the decrease of the coherence time have been evidenced, in particular an enhancement of the radiative recombination rate due to the resonant strong coupling between the dot and the one-dimensional optical mode. As a consequence, the emission couples very efficiently into the waveguide mode leading to an additional relaxation term of the excited state population.

http://xxx.lanl.gov/abs/1304.4756

Nonlinear dynamics of a driven nanomechanical single-electron transistor

We analyze the response of a nanomechanical resonator to an external drive when it is also coupled to a single-electron transistor (SET). The interaction between the SET electrons and the mechanical resonator depends on the amplitude of the mechanical motion leading to a strongly nonlinear response to the drive which is similar to that of a Duffing oscillator. We show that the average dynamics of the resonator is well described by a simple effective model which incorporates damping and frequency renormalization terms which are amplitude dependent. We also find that for a certain range of parameters the system displays interesting bistable dynamics in which noise arising from charge fluctuations causes the resonator to switch slowly between different dynamical states.

http://prb.aps.org/abstract/PRB/v87/i15/e155407

Controlling Fusion of Majorana Fermions in one-dimensional systems by Zeeman Field

We propose to realize Majorana fermions (MFs) on an edge of a two-dimensional topological insulator in the proximity with s-wave superconductors and in the presence of transverse exchange field h. It is shown that there appear a pair of MFs localized at two junctions and that a reverse in direction of h can lead to permutation of two MFs. With decreasing h, the MF states can either be fused or form one Dirac fermion on the {\pi}-junctions, exhibiting a topological phase transition. This characteristic can be used to detect physical states of MFs when they are transformed into Dirac fermions localized on the {\pi}-junction. A condition of decoupling two MFs is also given.

http://xxx.lanl.gov/abs/1304.4726

Understanding the structure of the first atomic contact in Gold

We have studied experimentally the phenomena of jump-to-contact (JC) and jump-out-of-contact (JOC) in gold electrodes. JC can be observed at the first contact when the two metals approach each other while JOC occurs in the last contact before breaking. When the indentation depth between the electrodes is limited to a certain value of conductance, a highly reproducible behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. Molecular dynamics simulations of this process show how the two metallic electrodes are shaped into tips of a well-defined crystallographic structure formed through a mechanical annealing mechanism. We report a detailed analysis of the atomic configurations obtained before contact and rupture of these stable structures and obtained their conductance using first-principlesquantum transport calculations. These results help us understand the values of conductance obtained experimentally in the JC and JOC phenomena and improve our understanding of atomic-sized contacts and the evolution of their structural characteristics.

http://xxx.lanl.gov/abs/1304.4774

Demonstration of Quantum Entanglement between a Single Electron Spin Confined to an InAs Quantum Dot and a Photon

The electron spin state of a singly charged semiconductor quantum dot has been shown to form a suitable single qubit for quantum computing architectures with fast gate times. A key challenge in realizing a useful quantum dot quantum computing architecture lies in demonstrating the ability to scale the system to many qubits. In this Letter, we report an all optical experimental demonstration of quantum entanglement between a single electron spin confined to a single charged semiconductor quantum dot and the polarization state of a photon spontaneously emitted from the quantum dot’s excited state. We obtain a lower bound on the fidelity of entanglement of 0.59±0.04, which is 84% of the maximum achievable given the timing resolution of available single photon detectors. In future applications, such as measurement-based spin-spin entanglement which does not require sub-nanosecond timing resolution, we estimate that this system would enable near ideal performance. The inferred (usable) entanglement generation rate is 3×103 s-1. This spin-photon entanglement is the first step to a scalable quantum dot quantum computing architecture relying on photon (flying) qubits to mediate entanglement between distant nodes of a quantum dot network.

http://prl.aps.org/abstract/PRL/v110/i16/e167401

Graphene MEMS: AFM Probe Performance Improvement

We explore the feasibility of growing a continuous layer of graphene in prepatterned substrates, like an engineered silicon wafer, and we apply this as a mold for the fabrication of AFM probes. This fabrication method proves the fabrication of SU-8 devices coated with graphene in a full-wafer parallel technology and with high yield. It also demonstrates that graphene coating enhances the functionality of SU-8 probes, turning them conductive and more resistant to wear. Furthermore, it opens new experimental possibilities such as studying graphene–graphene interaction at the nanoscale with the precision of an AFM or the exploration of properties in nonplanar graphene layers.

http://pubs.acs.org/doi/abs/10.1021/nn400557b

Ultrafast Charge Transfer at Monolayer Graphene Surfaces with Varied Substrate Coupling

The charge transfer rates of a localized excited electron to graphene monolayers with variable substrate coupling have been investigated by the core hole clock method with adsorbed argon. Expressed as charge transfer times, we find strong variations between 3 fs (on graphene “valleys” on Ru(0001)) to 16 fs (quasi-free graphene on SiC, O/Ru(0001), or SiO2/Ru). The values for the “hills” on Gr/Ru and on Gr/Pt(111) are in between, with the ratio 1.7 between the charge transfer times measured on “hills” and “valleys” of Gr/Ru. We discuss the results for Gr on metals in terms of hybridized Ru–C orbitals, which change with the relative Gr–Ru alignment and distance. The charge transfer on the decoupled graphene layers must represent the intrinsic coupling to the graphene empty π* states. Its low rate may be influenced by processes retarding the spreading of charge after transfer.

http://pubs.acs.org/doi/abs/10.1021/nn4008862

Ultrathin Two-Dimensional MnO2/Graphene Hybrid Nanostructures for High-Performance, Flexible Planar Supercapacitors

Planar supercapacitors have recently attracted much attention owing to their unique and advantageous design for 2D nanomaterials based energy storage devices. However, improving the electrochemical performance of planar supercapacitors still remains a great challenge. Here we report for the first time a novel, high-performance in-plane supercapacitor based on hybrid nanostructures of quasi-2D ultrathin MnO2/graphene nanosheets. Specifically, the planar structures based on the δ-MnO2 nanosheets integrated on graphene sheets not only introduce more electrochemically active surfaces for absorption/desorption of electrolyte ions, but also bring additional interfaces at the hybridized interlayer areas to facilitate charge transport during charging/discharging processes. The unique structural design for planar supercapacitors enables great performance enhancements compared to graphene-only devices, exhibiting high specific capacitances of 267 F/g at current density of 0.2 A/g and 208 F/g at 10 A/g and excellent rate capability and cycling stability with capacitance retention of 92% after 7000 charge/discharge cycles. Moreover, the high planar malleability of planar supercapacitors makes possible superior flexibility and robust cyclability, yielding capacitance retention over 90% after 1000 times of folding/unfolding. Ultrathin 2D nanomaterials represent a promising material platform to realize highly flexible planar energy storage devices as the power back-ups for stretchable/flexible electronic devices.

http://pubs.acs.org/doi/abs/10.1021/nl400600x

Márc. 30 - Ápr. 5. (2013)

Válogatta: Magyarkuti András""

Femtosecond switching of magnetism via strongly correlated spin–charge quantum excitations

Tianqi Li, Aaron Patz, Leonidas Mouchliadis, Jiaqiang Yan, Thomas A. Lograsso, Ilias E. Perakis & Jigang Wang

The technological demand to push the gigahertz (109 hertz) switching speed limit of today’s magnetic memory and logic devices into the terahertz (1012 hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation1. By analogy to femtosecond chemistry and photosynthetic dynamics2—in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states—femtosecond-laser-excited coherence between electronic states can switch magnetic order by ‘suddenly’ breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10−15 seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10−12 seconds) lattice-heating regime characterized by phase separation without threshold behaviour5, 6. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators and non-equilibrium phase transitions of strongly correlated electrons, in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.

Nature 496, 69–73 (04 April 2013) doi:10.1038/nature11934

Raman spectroscopy as a versatile tool for studying the properties of graphene

Andrea C. Ferrari & Denis M. Basko

Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp2-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene

Nature Nanotechnology 8, 235–246 (2013) doi:10.1038/nnano.2013.46

Terahertz spin current pulses controlled by magnetic heterostructures

T. Kampfrath, M. Battiato, P. Maldonado, G. Eilers, J. Nötzold, S. Mährlein, V. Zbarsky, F. Freimuth, Y. Mokrousov, S. Blügel, M. Wolf, I. Radu, P. M. Oppeneer & M. Münzenberg

In spin-based electronics, information is encoded by the spin state of electron bunches1, 2, 3, 4. Processing this information requires the controlled transport of spin angular momentum through a solid5, 6, preferably at frequencies reaching the so far unexplored terahertz regime7, 8, 9. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins10, 11, 12 from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter13 based on the inverse spin Hall effect14, 15, which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states16. Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters7, 8, 9.

Nature Nanotechnology 8, 256–260 (2013) doi:10.1038/nnano.2013.43

Quantum information: Atoms and circuits unite in silicon

Andrea Morello

While the size of silicon transistors in conventional computers shrinks towards the atomic scale, the quantum states of atoms and quantum dots in silicon are being investigated for quantum information processing.

Nature Nanotechnology 8, 233–234 (2013) doi:10.1038/nnano.2013.50

Probing from Both Sides: Reshaping the Graphene Landscape via Face-to-Face Dual-Probe Microscopy

Franz R. Eder , Jani Kotakoski , Katharina Holzweber , Clemens Mangler , Viera Skakalova , and Jannik C. Meyer

In two-dimensional samples, all atoms are at the surface and thereby exposed for probing and manipulation by physical or chemical means from both sides. Here, we show that we can access the same point on both surfaces of a few-layer graphene membrane simultaneously, using a dual-probe scanning tunneling microscopy (STM) setup. At the closest point, the two probes are separated only by the thickness of the graphene membrane. This allows us for the first time to directly measure the deformations induced by one STM probe on a free-standing membrane with an independent second probe. We reveal different regimes of stability of few-layer graphene and show how the STM probes can be used as tools to shape the membrane in a controlled manner. Our work opens new avenues for the study of mechanical and electronic properties of two-dimensional materials.

Nano Lett., Article ASAP DOI: 10.1021/nl3042799

Fluctuational internal Josephson eﬀect in topological insulator ﬁlm

D.K. Eﬁmkin and Yu.E. Lozovik

Tunneling between opposite surfaces of topological insulator thin ﬁlm populated by electrons and holes is considered. We predict considerable enhancement of tunneling conductivity by Cooper electron-hole pair fluctuations that are precursor of their Cooper pairing. Cooper pair ﬂuctuations lead to critical behavior of tunneling conductivity in vicinity of critical temperature with critical index ν = 2. If the pairing is suppressed by disorder the behavior of tunneling conductivity in vicinity of quantum phase transition is also critical with the index µ = 2. The eﬀect can be interpreted as ﬂuctuational internal Josephson eﬀect.

http://xxx.lanl.gov/pdf/1304.1436.pdf

Snapshots of non-equilibrium Dirac carrier distributions in graphene

Isabella Gierz, Jesse C. Petersen, Matteo Mitrano, Cephise Cacho, Edmond Turcu, Emma Springate, Alexander Stohr, Axel Kohler, Ulrich Starke, and Andrea Cavalleri

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point [1{10]. It has been proposed that even in the absence of a semiconducting bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths [4{6]. Furthermore, efficent carrier multiplication by impact ionization has been discussed in the context of light harvesting applications [7, 8]. However, all these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy and momentum dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme ultra-violet (EUV) pulses at 31.5 eV photon energy to directly probe the nonequilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples [11, 12], we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. While excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find indication for carrier multiplication, questioning the applicability of graphene for light harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the technique of choice to assess the suitability of new materials for optoelectronics, providing quantitatively accurate measurements of nonequilibrium carriers at all energies and wavevectors.

http://xxx.lanl.gov/pdf/1304.1389.pdf

Light emission and finite frequency shot noise in molecular junctions: from tunneling to contact

Jing-Tao Lu, Rasmus Bjerregaard Christensen, and Mads Brandbyge

Scanning tunneling microscope induced light emission from an atomic or molecular junction has been probed from the tunneling to contact regime in recent experiments. There, the intensity of the light emission shows strong correlation with the current/charge fluctuations at optical frequencies. We show that this is consistent with the established theory in the tunneling regime, by writing the finite- frequency shot noise as a sum of inelastic transitions between different electronic states. Based on this, we develop a practical scheme to perform calculations on realistic structures using Green's functions. The photon emission yields obtained re-produce the essential feature of the experiments.

http://xxx.lanl.gov/pdf/1304.0963.pdf

Spin Hall effect in a simple classical picture of spin forces

Sheng Ghee Tan, Mansoor B.A. Jalil

Spin Hall effect (SHE) in a 2D-Rashba system has been treated in the spin-dependent precession and the time-space gauge approaches, both yielding SHE conductivity of spin transverse force provide a heuristic but not a quantifiable indication of SHE. In this paper, we provide a more complete description of the SHE using the Heisenberg approach, unifying the Yang-Mills force, the Heisenberg spin force, and the SHE under the classical notion of forces and accelerations. Central to this paper is the spin force equations that are satisfied by both and , the Yang-Mills, and the Heisenberg spin forces. By linking to the spin forces, one sees that the physics of SHE in a 2D-Rashba system is also a simple classical Lorentz force picture.

http://xxx.lanl.gov/pdf/1304.0830.pdf

Vortex states and Majorana fermions in spin-orbit coupled semiconductor-superconductor hybrid structures

Kristofer Bjornson and Annica M. Black-Schaffer

We study the energy spectrum of a vortex core in a two-dimensional semiconductor with Rashba spin-orbit interaction and proximity-coupled to a conventional superconductor and a ferromagnetic insulator. We perform self-consistent calculations using the microscopic tight-binding Bogoliubov-de Gennes method on a lattice and conﬁrm the existence of Majorana fermions in the non-trivial topological phase. We also ﬁnd two different topologically trivial superconducting phases, only differing in the type of vortex core structure they support and separated by a normal fermionic zero-energy excitation. Furthermore, we ﬁnd an asymmetry in the energy spectrum with respect to both Zeeman splitting and vortex rotation direction and explain its physical origin.

http://xxx.lanl.gov/pdf/1304.0981.pdf

Márc. 22 - 29. (2013)

Válogatta: Pósa László""

Atomically Wired Molecular Junctions: Connecting a Single Organic Molecule by Chains of Metal Atoms

Tamar Yelin, Ran Vardimon, Natalia Kuritz, Richard Korytar,́ Alexei Bagrets, Ferdinand Evers, Leeor Kronik and Oren Tal

Using a break junction technique, we ﬁnd a clear signature for the formation of conducting hybrid junctions composed of a single organic molecule (benzene, naphthalene, or anthracene) connected to chains of platinum atoms. The hybrid junctions exhibit metallic-like conductance (∼0.1−1G0), which is rather insensitive to further elongation by additional atoms. At low bias voltage the hybrid junctions can be elongated signiﬁcantly beyond the length of the bare atomic chains. Ab initio calculations reveal that benzene based hybrid junctions have a signiﬁcant binding energy and high structural ﬂexibility that may contribute to the survival of the hybrid junction during the elongation process. The fabrication of hybrid junctions opens the way for combining the diﬀerent properties of atomic chains and organic molecules to realize a new class of atomic scale interfaces.

http://pubs.acs.org/doi/pdf/10.1021/nl304702z

Anomalous Modulation of a Zero-Bias Peak in a Hybrid Nanowire-Superconductor Device

A. D. K. Finck, D. J. Van Harlingen, P. K. Mohseni, K. Jung and X. Li

We report on transport measurements of an InAs nanowire coupled to niobium nitride leads at high magnetic ﬁelds. We observe a zero-bias anomaly (ZBA) in the differential conductance of the nanowire for certain ranges of magnetic ﬁeld and chemical potential. The ZBA can oscillate in width with either the magnetic ﬁeld or chemical potential; it can even split and re-form. We discuss how our results relate to recent predictions of hybridizing Majorana fermions in semiconducting nanowires, while considering more mundane explanations.

http://prl.aps.org/pdf/PRL/v110/i12/e126406

Broken Symmetry Quantum Hall States in Dual-Gated ABA Trilayer Graphene

Yongjin Lee, Jairo Velasco Jr, David Tran, Fan Zhang, W. Bao, Lei Jing, Kevin Myhro, Dmitry Smirnov and Chun Ning Lau

ABA-stacked trilayer graphene is a unique 2D electron system with mirror reﬂection symmetry and unconventional quantum Hall eﬀect. We present low-temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at ﬁlling factors ν =−8,−2, 2, 6, and 10, which is in agreement with the full-parameter tight binding calculations. In high magnetic ﬁelds, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Landau level (LL). Under an out-ofplane electric ﬁeld E ⊥, we observe degeneracy breaking and transitions between QH plateaus. Interestingly, depending on its direction, E ⊥ selectively breaks the LL degeneracies in the electron-doped or holedoped regimes. Our results underscore the rich interaction-induced phenomena in trilayer graphene

http://pubs.acs.org/doi/pdf/10.1021/nl4000757

A Graphene-Based Hot Electron Transistor

Sam Vaziri, Grzegorz Lupina, Christoph Henkel, Anderson D. Smith, Mikael Östling, Jarek Dabrowski, Gunther Lippert, Wolfgang Mehr and Max C. Lemme

We experimentally demonstrate DC functionality of graphene-based hot electron transistors, which we call graphene base transistors (GBT). The fabrication scheme is potentially compatible with silicon technology and can be carried out at the wafer scale with standard silicon technology. The state of the GBTs can be switched by a potential applied to the transistor base, which is made of graphene. Transfer characteristics of the GBTs show ON/OFF current ratios exceeding 10^4.

http://pubs.acs.org/doi/pdf/10.1021/nl304305x

Molecular electronics: Reflections on charge transport

Georg Heimel, Jean-Luc Brédas, Jean-Luc Brédas

By varying the distance between two electrodes bridged by a single molecule, the interaction between charges passing through the molecular junction and their mirror images in the metal contacts can be observed.

http://www.nature.com/nnano/journal/v8/n4/full/nnano.2013.42.html

Large tunable image-charge effects in single-molecule junctions

Mickael L. Perrin, Christopher J. O. Verzijl, Christian A. Martin, Joseph M. Thijssen, Herre S. J. van der Zant & Diana Dulić, Ahson J. Shaikh, Rienk Eelkema & Jan H. van Esch, Jan M. van Ruitenbeek

Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.

http://www.nature.com/nnano/journal/v8/n4/full/nnano.2013.26.html

Terahertz quantum Hall effect of Dirac fermions in a topological insulator

A. M. Shuvaev, G. V. Astakhov, G. Tkachov, C. Brune, H. Buhmann,L. W. Molenkamp and A. Pimenov

Using terahertz spectroscopy in external magnetic ﬁelds, we investigate the low-temperature charge dynamics of strained HgTe, a three-dimensional topological insulator. In resonator experiments, we observe quantum Hall oscillations at terahertz frequencies, which offer direct access to the unusual electrodynamic properties of the surface states of topological insulators. The 2D density estimated from the period of the quantum Hall oscillations agrees well with dc transport experiments on the topological surface state. The Dirac character of the surface state is further evidenced by the observation of the characteristic Berry phase in the dependence of the Landau levels on magnetic ﬁeld

http://prb.aps.org/pdf/PRB/v87/i12/e121104

Phase-locked magnetoconductance oscillations as a probe of Majorana edge states

M. Diez, I. C. Fulga, D. I. Pikulin, M. Wimmer, A. R. Akhmerov and C. W. J. Beenakker

We calculate the Andreev conductance of a superconducting ring interrupted by a ﬂux-biased Josephson junction, searching for electrical signatures of circulating edge states. Two-dimensional pair potentials of spinsingletd-wave and spin-tripletp-wave symmetry support, respectively, (chiral) Dirac modes and (chiral or helical) Majorana modes. These produce h/e-periodic magnetoconductance oscillations of amplitude �(e^2/h)N^(−1/2), measured via an N-mode point contact at the inner or outer perimeter of the grounded ring. For Dirac modes the oscillations in the two contacts are independent, while for an unpaired Majorana mode they are phase locked by a topological phase transition at the Josephson junction.

http://prb.aps.org/pdf/PRB/v87/i12/e125406

Márc. 15 - 21. (2013)

Válogatta: Scherübl Zoltán

Quantum Hall effect in graphene with superconducting electrodes

Peter Rickhaus, Markus Weiss, Laurent Marot, Christian Schönenberger

We have realized an integer quantum Hall system with superconducting contacts by connecting graphene to niobium electrodes. Below their upper critical field of 4 tesla, an integer quantum Hall effect coexists with superconductivity in the leads, but with a plateau conductance that is larger than in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the graphene-superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor, and is less pronounced on the first plateau, due to the special nature of the zero energy Landau level in monolayer graphene.

http://arxiv.org/abs/1303.3394

Enhancement of the Electrical Properties of Graphene Grown by Chemical Vapor Deposition via Controlling the Effects of Polymer Residue

Ji Won Suk †, Wi Hyoung Lee †‡, Jongho Lee §, Harry Chou †, Richard D. Piner †, Yufeng Hao †, Deji Akinwande §, and Rodney S. Ruoff *†

Residual polymer (here, poly(methyl methacrylate), PMMA) left on graphene from transfer from metals or device fabrication processes affects its electrical and thermal properties. We have found that the amount of polymer residue left after the transfer of chemical vapor deposited (CVD) graphene varies depending on the initial concentration of the polymer solution, and this residue influences the electrical performance of graphene field-effect transistors fabricated on SiO2/Si. A PMMA solution with lower concentration gave less residue after exposure to acetone, resulting in less p-type doping in graphene and higher charge carrier mobility. The electrical properties of the weakly p-doped graphene could be further enhanced by exposure to formamide with the Dirac point at nearly zero gate voltage and a more than 50% increase of the room-temperature charge carrier mobility in air. This can be attributed to electron donation to graphene by the −NH2 functional group in formamide that is absorbed in the polymer residue. This work provides a route to enhancing the electrical properties of CVD-grown graphene even when it has a thin polymer coating.

http://pubs.acs.org/doi/abs/10.1021/nl304420b

Controlling the Orientation, Edge Geometry, and Thickness of Chemical Vapor Deposition Graphene

Adrian T. Murdock †, Antal Koos †, T. Ben Britton †, Lothar Houben ‡, Tim Batten §, Tong Zhang §, Angus J. Wilkinson †, Rafal E. Dunin-Borkowski ‡, Christina E. Lekka , and Nicole Grobert *†

We report that the shape, orientation, edge geometry, and thickness of chemical vapor deposition graphene domains can be controlled by the crystallographic orientations of Cu substrates. Under low-pressure conditions, single-layer graphene domains align with zigzag edges parallel to a single 101 direction on Cu(111) and Cu(101), while bilayer domains align to two directions on Cu(001). Under atmospheric pressure conditions, hexagonal domains also preferentially align. This discovery can be exploited to generate high-quality, tailored graphene with controlled domain thickness, orientations, edge geometries, and grain boundaries.

http://pubs.acs.org/doi/abs/10.1021/nn3049297

Noise in electromigrated nanojunctions

P. J. Wheeler, Ruoyu Chen., D. Natelson

Noise measurements are a probe beyond simple electronic transport that can reveal additional information about electronic correlations and inelastic processes. Here we report noise measurements in individual electromigrated nanojunctions, examining the evolution from the many channel regime to the tunneling regime, using a radio frequency technique. While we generally observe the dependence of noise on bias expected for shot noise, in approximately 12% of junction configurations we find discrete changes in the bias dependence at threshold values of the bias, consistent with electronic excitation of local vibrational modes. Moreover, with some regularity we find significant mesoscopic variation in the magnitude of the noise in particular junctions even with small changes in the accompanying conductance. In another $\sim$17% of junctions we observe pronounced asymmetries in the inferred noise magnitude as a function of bias polarity, suggesting that investigators should be concerned about current-driven ionic motion in the electrodes even at biases well below those used for deliberate electromigration.

http://arxiv.org/abs/1303.4131

One-Dimensional Quantum Confinement Effect Modulated Thermoelectric Properties in InAs Nanowires

Yuan Tian, Mohammed R. Sakr, Jesse M. Kinder, Dong Liang, Michael J. MacDonald, Richard L.J. Qiu, Hong-Jun Gao, Xuan P.A. Gao

We report electrical conductance and thermopower measurements on InAs nanowires synthesized by chemical vapor deposition. Gate modulation of the thermopower of individual InAs nanowires with diameter around 20nm is obtained over T=40 to 300K. At low temperatures (T< ~100K), oscillations in the thermopower and power factor concomitant with the stepwise conductance increases are observed as the gate voltage shifts the chemical potential of electrons in InAs nanowire through quasi-one-dimensional (1D) sub-bands. This work experimentally shows the possibility to modulate semiconductor nanowire's thermoelectric properties through the peaked 1D electronic density of states in the diffusive transport regime, a long-sought goal in nanostructured thermoelectrics research. Moreover, we point out the importance of scattering (or disorder) induced energy level broadening in smearing out the 1D confinement enhanced thermoelectric power factor at practical temperatures (e.g. 300K).

http://arxiv.org/abs/1303.3838

Unpaired Floquet Majorana fermions without magnetic fields

Andres A. Reynoso1 and Diego Frustaglia

Quantum wires subject to the combined action of spin-orbit and Zeeman coupling in the presence of s-wave pairing potentials (superconducting proximity effect in semiconductors or superfluidity in cold atoms) are one of the most promising systems for the developing of topological phases hosting Majorana fermions. The breaking of time-reversal symmetry is essential for the appearance of unpaired Majorana fermions. By implementing a time-dependent spin rotation, we show that the standard magnetostatic model maps into a nonmagnetic one where the breaking of time-reversal symmetry is guaranteed by a periodical change of the spin-orbit coupling axis as a function of time. This suggests the possibility of developing the topological superconducting state of matter driven by external forces in the absence of magnetic fields and magnetic elements. From a practical viewpoint, the scheme avoids the disadvantages of conjugating magnetism and superconductivity, even though the need of a high-frequency driving of spin-orbit coupling may represent a technological challenge. We describe the basic properties of this Floquet system by showing that finite samples host unpaired Majorana fermions at their edges despite the fact that the bulk Floquet quasienergies are gapless and that the Hamiltonian at each instant of time preserves time-reversal symmetry. Remarkably, we identify the mean energy of the Floquet states as a topological indicator. We additionally show that the localized Floquet Majorana fermions are robust under local perturbations. Our results are supported by complementary numerical Floquet simulations.

http://prb.aps.org/abstract/PRB/v87/i11/e115420

Márc. 2 - 14. (2013)

Válogatta: Gubicza Ági

Enhancement of the Electrical Properties of Graphene Grown by Chemical Vapor Deposition via Controlling the Effects of Polymer Residue

Ji Won Suk, Wi Hyoung Lee, Jongho Lee, Harry Chou, Richard D. Piner, Yufeng Hao, Deji Akinwande, and Rodney S. Ruoff

Residual polymer (here, poly(methyl methacrylate), PMMA) left on graphene from transfer from metals or device fabrication processes affects its electrical and thermal properties. We have found that the amount of polymer residue left after the transfer of chemical vapor deposited (CVD) graphene varies depending on the initial concentration of the polymer solution, and this residue influences the electrical performance of graphene field-effect transistors fabricated on SiO2/Si. A PMMA solution with lower concentration gave less residue after exposure to acetone, resulting in less p-type doping in graphene and higher charge carrier mobility. The electrical properties of the weakly p-doped graphene could be further enhanced by exposure to formamide with the Dirac point at nearly zero gate voltage and a more than 50% increase of the room-temperature charge carrier mobility in air. This can be attributed to electron donation to graphene by the −NH2 functional group in formamide that is absorbed in the polymer residue. This work provides a route to enhancing the electrical properties of CVD-grown graphene even when it has a thin polymer coating.

http://pubs.acs.org/doi/abs/10.1021/nl304420b

Graphene Drape Minimizes the Pinning and Hysteresis of Water Drops on Nanotextured Rough Surfaces

Eklavya Singh, Abhay V. Thomas, Rahul Mukherjee, Xi Mi, Farzad Houshmand, Yoav Peles, Yunfeng Shi, and Nikhil Koratkar

Previous studies of the interaction of water with graphene-coated surfaces have been limited to flat (smooth) surfaces. Here we created a rough surface by nanopatterning and then draped the surface with a single-layer graphene sheet. We found that the ultrasheer graphene drape prevents the penetration of water into the textured surface thereby drastically reducing the contact angle hysteresis (which is a measure of frictional energy dissipation) and preventing the liquid contact line from getting pinned to the substrate. This has important technological implications since the main obstacle to the motion of liquid drops on rough surfaces is contact angle hysteresis and contact line pinning. Graphene drapes could therefore enable enhanced droplet mobility which is required in a wide range of applications in micro and nanofluidics. Compared to polymer coatings that could fill the cavities between the nano/micropores or significantly alter the roughness profile of the substrate, graphene provides the thinnest (i.e., most sheer) and most conformal drape that is imaginable. Despite its extreme thinness, the graphene drape is mechanically robust, chemically stable, and offers high flexibility and resilience which can enable it to reliably drape arbitrarily complex surface topologies. Graphene drapes may therefore provide a hitherto unavailable ability to tailor the dynamic wettability of surfaces for a variety of applications.

http://pubs.acs.org/doi/abs/10.1021/nn400466t

Majorana's wires

Marcel Franz

Experiments on nanowires have shown evidence of solid-state analogues of the particles predicted by Ettore Majorana more than 70 years ago. Although stronger confirmation is still to come, these first observations have already fuelled expectations of fundamental results and potential applications in quantum information technology.

http://www.nature.com/nnano/journal/v8/n3/full/nnano.2013.33.html

Molecular spintronics: Stretch for a moment

The spin of a single-molecule magnet is coupled to the vibrational motion of a single carbon nanotube.

http://www.nature.com/nnano/journal/v8/n3/full/nnano.2013.27.html

Large tunable image-charge effects in single-molecule junctions

Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.26.html

Molecular electronics: Reflections on charge transport

By varying the distance between two electrodes bridged by a single molecule, the interaction between charges passing through the molecular junction and their mirror images in the metal contacts can be observed.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.42.html

Full-counting statistics for molecular junctions: Fluctuation theorem and singularities

Y. Utsumi, O. Entin-Wohlman, A. Ueda, and A. Aharony

We study the full-counting statistics of charges transmitted through a single-level quantum dot weakly coupled to a local Einstein phonon which causes fluctuations in the dot energy. An analytic expression for the cumulant generating function, accurate up to second order in the electron-phonon coupling and valid for finite voltages and temperatures, is obtained in the extended wide-band limit. The result accounts for nonequilibrium phonon distributions induced by the source-drain bias voltage, and concomitantly satisfies the fluctuation theorem. Extending the counting field to the complex plane, we investigate the locations of possible singularities of the cumulant generating function, and exploit them to identify regimes in which the electron transfer is affected differently by the coupling to the phonons. Within a large-deviation analysis, we find a kink in the probability distribution, analogous to a first-order phase transition in thermodynamics, which would be a unique hallmark of the electron-phonon correlations. This kink reflects the fact that although inelastic scattering by the phonons once the voltage exceeds their frequency can scatter electrons opposite to the bias, this will never generate current flowing against the bias at zero temperature, in accordance with the fluctuation theorem.

http://prb.aps.org/abstract/PRB/v87/i11/e115407

Quantum Monte Carlo study of nonequilibrium transport through a quantum dot coupled to normal and superconducting leads

Akihisa Koga

We investigate the nonequilibrium phenomena through a quantum dot coupled to normal and superconducting leads using a weak-coupling continuous-time Monte Carlo method. Calculating the time evolution of the particle number, double occupancy, and pairing correlation at the quantum dot, we discuss how the system approaches the steady state. We also deduce the steady current through the quantum dot beyond the linear-response region. It is clarified that the interaction decreases the current in the Kondo-singlet dominant region. On the other hand, when the quantum dot is tightly coupled to the superconducting lead, the current is increased by the introduction of a Coulomb interaction, which originates from the competition between the Kondo and proximity effects. Transient currents induced by the interaction quench are also addressed.

http://prb.aps.org/abstract/PRB/v87/i11/e115409

Quantum Hall ferromagnetic phases in the Landau level N=0 of a graphene bilayer

J. Lambert and R. Côté

In a Bernal-stacked graphene bilayer, an electronic state in Landau level N=0 is described by its guiding-center index X (in the Landau gauge) and by its valley, spin, and orbital indices ξ=±K, σ=±1, and n=0,1. When Coulomb interaction is taken into account, the chiral two-dimensional electron gas (C2DEG) in this system can support a variety of quantum Hall ferromagnetic ground states where the spins and/or valley pseudospins and/or orbital pseudospins collectively align in space. In this work, we give a comprehensive account of the phase diagram of the C2DEG at integer filling factors ν∈[−3,3] in Landau level N=0 when an electrical potential difference ΔB between the two layers is varied. We consider states with or without layer, spin, or orbital coherence. For each phase, we discuss the behavior of the transport gap as a function of ΔB, the spectrum of collective excitations, and the optical absorption due to orbital pseudospin-wave modes. We also study the effect of an external in-plane electric field on a coherent state that has both valley and spin coherence and show that it is possible, in such a state, to control the spin polarization by varying the strength of the external in-plane electric field.

http://prb.aps.org/abstract/PRB/v87/i11/e115415

Febr. 23- Márc. 1. (2013)

Válogatta: Tóvári Endre

Coherence and Indistinguishability of Single Electrons Emitted by Independent Sources

E. Bocquillon, V. Freulon, J.-M Berroir, P. Degiovanni, B. Plaçais, A. Cavanna, Y. Jin, G. Fève

The on-demand emission of coherent and indistinguishable electrons by independent synchronized sources is a challenging task of quantum electronics, in particular regarding its application for quantum information processing. Using two independent on-demand electron sources, we triggered the emission of two single-electron wave packets at different inputs of an electronic beam splitter. Whereas classical particles would be randomly partitioned by the splitter, we observed two-particle interference resulting from quantum exchange. Both electrons, emitted in indistinguishable wave packets with synchronized arrival time on the splitter, exited in different outputs as recorded by the low-frequency current noise. The demonstration of two-electron interference provides the possibility of manipulating coherent and indistinguishable single-electron wave packets in quantum conductors.

http://www.sciencemag.org/content/339/6123/1054.abstract?sid=c605ea04-9b45-44fd-b5f7-ce60fe059f91

Two Indistinguishable Electrons Interfere in an Electronic Device

Christian Schönenberger

In quantum mechanics, particles can be prepared in entangled states, so that measurement of a property on one particle determines the outcome for the other, no matter how far apart the particles may be. This "spooky action at distance" was demonstrated first with photons (1). One goal of condensed-matter physics has been to replicate quantum optics experiments with electrons (2). For example, the Hong-Ou-Mandel (HOM) experiment (3) can determine if two photons are indistinguishable—meaning that they have the same wavelength and polarization, and that they can become entangled if they overlap during propagation, as can happen at a beam splitter (a semitransparent mirror; see the figure, panels A and B). An electronic device that could demonstrate indistinguishability of electrons would be useful for quantum computing applications. On page 1054 of this issue, Bocquillon et al. (4) demonstrate such an analog of the HOM experiment with two electrons, generated from two different single-electron sources, colliding in the equivalent of a beam splitter in a single device.

http://www.sciencemag.org/content/339/6123/1041.summary?sid=c605ea04-9b45-44fd-b5f7-ce60fe059f91

Differentiation of Complex Vapor Mixtures Using Versatile DNA–Carbon Nanotube Chemical Sensor Arrays

Nicholas J. Kybert , Mitchell B. Lerner , Jeremy S. Yodh , George Preti , and A. T. Charlie Johnson

Vapor sensors based on functionalized carbon nanotubes (NTs) have shown great promise, with high sensitivity conferred by the reduced dimensionality and exceptional electronic properties of the NT. Critical challenges in the development of NT-based sensor arrays for chemical detection include the demonstration of reproducible fabrication methods and functionalization schemes that provide high chemical diversity to the resulting sensors. Here, we outline a scalable approach to fabricating arrays of vapor sensors consisting of NT field effect transistors functionalized with single-stranded DNA (DNA-NT). DNA-NT sensors were highly reproducible, with responses that could be described through equilibrium thermodynamics. Target analytes were detected even in large backgrounds of volatile interferents. DNA-NT sensors were able to discriminate between highly similar molecules, including structural isomers and enantiomers. The sensors were also able to detect subtle variations in complex vapors, including mixtures of structural isomers and mixtures of many volatile organic compounds characteristic of humans.

http://pubs.acs.org/doi/abs/10.1021/nn400359c

Investigation of MoS2 and Graphene Nanosheets by Magnetic Force Microscopy

Hai Li , Xiaoying Qi , Jumiati Wu , Zhiyuan Zeng , Jun Wei , and Hua Zhang

For the first time, magnetic force microscopy (MFM) is used to characterize the mechanically exfoliated single- and few-layer MoS2 and graphene nanosheets. By analysis of the phase and amplitude shifts, the magnetic response of MoS2 and graphene nanosheets exhibits the dependence on their layer number. However, the solution-processed single-layer MoS2 nanosheet shows the reverse magnetic signal to the mechanically exfoliated one, and the graphene oxide nanosheet has not shown any detectable magnetic signal. Importantly, graphene and MoS2 flakes become nonmagnetic when they exceed a certain thickness.

http://pubs.acs.org/doi/abs/10.1021/nn400443u

Superconducting Spin Switch with Infinite Magnetoresistance Induced by an Internal Exchange Field

Bin Li, Niklas Roschewsky, Badih A. Assaf, Marius Eich, Marguerite Epstein-Martin, Don Heiman, Markus Münzenberg, and Jagadeesh S. Moodera

A theoretical prediction by de Gennes suggests that the resistance in a FI/S/FI (where FI is a ferromagnetic insulator, and S is a superconductor) structure will depend on the magnetization direction of the two FI layers. We report a magnetotransport measurement in a EuS/Al/EuS structure, showing that an infinite magnetoresistance can be produced by tuning the internal exchange field at the FI/S interface. This proximity effect at the interface can be suppressed by an Al2O3 barrier as thin as 0.3 nm, showing the extreme confinement of the interaction to the interface giving rise to the demonstrated phenomena.

http://prl.aps.org/abstract/PRL/v110/i9/e097001

Coherent edge mixing and interferometry in quantum Hall bilayers

Stefano Roddaro, Luca Chirolli, Fabio Taddei, Marco Polini, and Vittorio Giovannetti

We discuss the implementation of a beam splitter for electron waves in a quantum Hall bilayer. Our architecture exploits interlayer tunneling to mix edge states belonging to different layers. We discuss the basic working principle of the proposed coherent edge mixer, possible interferometric implementations based on existing semiconductor-heterojunction technologies, and advantages with respect to canonical quantum Hall interferometers based on quantum point contacts.

http://prb.aps.org/abstract/PRB/v87/i7/e075321

Proximity-induced giant spin-orbit interaction in epitaxial graphene on a topological insulator

Kyung-Hwan Jin and Seung-Hoon Jhi

Heterostructures of Dirac materials such as graphene and topological insulators provide interesting platforms to explore exotic quantum states of electrons in solids. Here we study the electronic structure of the graphene-Sb2Te3 heterostructure using density functional theory and tight-binding methods. We show that the epitaxial graphene on Sb2Te3 turns into the quantum spin-Hall phase due to its proximity to the topological-insulating Sb2Te3. It is found that the epitaxial graphene develops a giant spin-orbit gap of about ∼20 meV, which is about three orders of magnitude larger than that of pristine graphene. We discuss the origin of such enhancement of the spin-orbit interaction and possible outcomes of the spin-Hall phase in graphene.

http://prb.aps.org/abstract/PRB/v87/i7/e075442

Shubnikov–de Haas oscillations in the bulk Rashba semiconductor BiTeI

C. Bell, M. S. Bahramy, H. Murakawa, J. G. Checkelsky, R. Arita, Y. Kaneko, Y. Onose, M. Tokunaga, Y. Kohama, N. Nagaosa, Y. Tokura, and H. Y. Hwang

Bulk magnetoresistance quantum oscillations are observed in high quality single crystal samples of BiTeI. This compound shows an extremely large internal spin-orbit coupling, associated with the polarity of the alternating Bi, Te, and I layers perpendicular to the c axis. The corresponding areas of the inner and outer Fermi surfaces around the A point show good agreement with theoretical calculations, demonstrating that the intrinsic bulk Rashba-type splitting is nearly 360 meV, comparable to the largest spin-orbit coupling generated in heterostructures and at surfaces.

http://prb.aps.org/abstract/PRB/v87/i8/e081109

Calibrating atomic-scale force sensors installed at the tip apex of a scanning tunneling microscope

G. Kichin, C. Wagner, F. S. Tautz, and R. Temirov

Scanning tunneling microscopy (STM) tips decorated with either a single carbon monoxide molecule or a single xenon atom are characterized by simultaneous force and conductance measurements using a combined low-temperature noncontact atomic force and scanning tunneling microscope (NC-AFM/STM). It is shown that in both cases the particle decorating the tip simultaneously performs the function of an atomic-scale force sensor and transducer which couples the short-range force acting on the tip to the tunneling conductance of the junction. On the basis of the experimental data, two distinct coupling regimes are identified; in one of them the force sensor-transducer function is calibrated quantitatively.

http://prb.aps.org/abstract/PRB/v87/i8/e081408

Scanning gate microscopy of localized states in wide graphene constrictions

Andrei G. F. Garcia, Markus König and David Goldhaber-Gordon, Kathryn Todd

In graphene nanoconstrictions, charge puddles due to substrate-induced potential inhomogeneities can form quantum dots. We use a scanning gate microscope to probe a short, wide graphene constriction and observe a set of ring-shaped resonances in maps of the graphene conductance as a function of the local potential perturbation, a signature of quantum dot-mediated transport. By varying the scanning tip height and applied tip voltage, we observe the screening effect of the nearby graphene. Introducing a simple capacitive model to account for screening, we model the conduction of the graphene constriction and how the tip gates the quantum dots. We identify the locations of some of the quantum dots and extract their parameters.

http://prb.aps.org/abstract/PRB/v87/i8/e085446

Majorana Fermions in Semiconductor Nanowires: Fundamentals, Modeling, and Experiment

Tudor D. Stanescu, Sumanta Tewari

After a recent series of rapid and exciting developments, the long search for the Majorana fermion - the elusive quantum entity at the border between particles and antiparticles - has produced the first positive experimental results, but is not over yet. Originally proposed by E. Majorana in the context of particle physics, Majorana fermions have a condensed matter analog in the zero-energy bound states emerging in topological superconductors. A promising route to engineering topological superconductors capable of hosting Majorana zero modes consists of proximity coupling semiconductor thin films or nanowires with strong spin-orbit interaction to conventional s-wave superconductors in the presence of an external Zeeman field. The Majorana zero mode is predicted to emerge above a certain critical Zeeman field as a zero-energy state localized near the order parameter defects, viz., vortices for thin films and wire-ends for the nanowire. These Majorana bound states are expected to manifest non--Abelian quantum statistics, which makes them ideal building blocks for fault--tolerant topological quantum computation. This review provides an update on current status of the search for Majorana fermions in semiconductor nanowires by focusing on the recent developments, in particular the period following the first reports of experimental signatures consistent with the realization of Majorana bound states in semiconductor nanowire--superconductor hybrid structures. We start with a discussion of the fundamental aspects of the subject, followed by considerations on the realistic modeling which is a critical bridge between theoretical predictions based on idealized conditions and the real world, as probed experimentally. The last part is dedicated to a few intriguing issues that were brought to the fore by the recent encouraging experimental advances.

http://arxiv.org/abs/1302.5433

Febr. 16- Febr. 22. (2013)

Válogatta: Balogh Zoltán

Spin-resolved Andreev levels in hybrid superconductor-semiconductor nanostructures

Eduardo J. H. Lee, Xiaocheng Jiang, Manuel Houzet, Ramon Aguado, Charles M. Lieber, Silvano De Franceschi

The combination of superconductors and low-dimensional conductors embodies a rich, yet largely unexplored physics.

In this hybrid system, macroscopic properties enforced by superconductivity can be controlled through electrically

tunable microscopic degrees of freedom, inherent to a relatively small number of confined electrons. Here we

consider the prototypical case of a quantum dot (QD) coupled strongly to a superconductor (S) and weakly to a

normal-metal (N) tunnel probe. We investigate the magnetic properties of the lowest-energy, sub-gap states, which

are governed by a competition between superconducting pairing and Coulomb repulsion. In a magnetic field, only when

the ground state is a spin singlet, can the Zeeman splitting of the (excited) doublet be revealed by tunnel

spectroscopy. The splitting is strongly influenced by a level-repulsion effect with the continuum of quasi-particle

states; and it can induce a quantum phase transition (QPT) to a spin-polarized state. Our experimental results,

supported by theory, hold relevance for current research on quantum-information devices and Majorana fermions in

hybrid nanostructures.

http://arxiv.org/abs/1302.2611

Electrically tunable transverse magnetic focusing in graphene

Thiti Taychatanapat, Kenji Watanabe, Takashi Taniguchi & Pablo Jarillo-Herrero

Electrons in a periodic lattice can propagate without scattering for macroscopic distances despite the presence of

the non-uniform Coulomb potential due to the nuclei1. Such ballistic motion of electrons allows the use of a

transverse magnetic field to focus electrons. This phenomenon, known as transverse magnetic focusing (TMF), has been

used to study the Fermi surface of metals and semiconductor heterostructures4, as well as to investigate Andreev

reflection and spin–orbit interaction, and to detect composite fermions. Here we report on the experimental

observation of TMF in high-mobility mono-, bi- and tri-layer graphene devices. The ability to tune the graphene

carrier density enables us to investigate TMF continuously from the hole to the electron regime and analyse the

resulting focusing fan. Moreover, by applying a transverse electric field to tri-layer graphene, we use TMF as a

ballistic electron spectroscopy method to investigate controlled changes in the electronic structure of a material.

Finally, we demonstrate that TMF survives in graphene up to 300 K, by far the highest temperature reported for any

system, opening the door to new room-temperature applications based on electron-optics.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2549.html

Atom-Atom Entanglement by Single-Photon Detection

L. Slodička, G. Hétet, N. Röck, P. Schindler, M. Hennrich, and R. Blatt

A scheme for entangling distant atoms is realized, as proposed in the seminal paper by [ C. Cabrillo et al. Phys.

Rev. A 59 1025 (1999)]. The protocol is based on quantum interference and detection of a single photon scattered

from two effectively one meter distant laser cooled and trapped atomic ions. The detection of a single photon

heralds entanglement of two internal states of the trapped ions with high rate and with a fidelity limited mostly by

atomic motion. Control of the entangled state phase is demonstrated by changing the path length of the single-photon

interferometer.

http://prl.aps.org/abstract/PRL/v110/i8/e083603

Coherent Optical Memory with High Storage Efficiency and Large Fractional Delay

Yi-Hsin Chen, Meng-Jung Lee, I-Chung Wang, Shengwang Du, Yong-Fan Chen, Ying-Cheng Chen, and Ite A. Yu

A high-storage efficiency and long-lived quantum memory for photons is an essential component in long-distance

quantum communication and optical quantum computation. Here, we report a 78% storage efficiency of light pulses in a

cold atomic medium based on the effect of electromagnetically induced transparency. At 50% storage efficiency, we

obtain a fractional delay of 74, which is the best up-to-date record. The classical fidelity of the recalled pulse

is better than 90% and nearly independent of the storage time, as confirmed by the direct measurement of phase

evolution of the output light pulse with a beat-note interferometer. Such excellent phase coherence between the

stored and recalled light pulses suggests that the current result may be readily applied to single photon wave

packets. Our work significantly advances the technology of electromagnetically induced transparency–based optical

memory and may find practical applications in long-distance quantum communication and optical quantum computation.

http://prl.aps.org/abstract/PRL/v110/i8/e083601

Molecular Doping and Band-Gap Opening of Bilayer Graphene

Alexander J. Samuels and J. David Carey

The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and

opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic

and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or

p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers

and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a

slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/1013 cm–2, and p-type dopants

where it varies as 40 meV/1013 cm–2. Decamethylcobaltocene (DMC, n-type) and

3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing

the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8–4 μm region of the

spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated

from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene

devices. A key advantage of using molecular adsorption with π electron containing molecules is that the high binding

energy can induce a permanent band gap and open up possible uses of bilayer graphene in mid-infrared photonic or

electronic device applications.

http://pubs.acs.org/doi/abs/10.1021/nn400340q

Observation of Near-Field Dipolar Interactions Involved in a Metal Nanoparticle Chain Waveguide

A. Apuzzo, M. Février, R. Salas-Montiel, A. Bruyant, A. Chelnokov, G. Lérondel, B. Dagens, and S. Blaize

We present near-field measurements of transverse plasmonic wave propagation in a chain of gold elliptical

nanocylinders fed by a silicon refractive waveguide at optical telecommunication wavelengths. Eigenmode amplitude

and phase imaging by apertureless scanning near-field optical microscopy allows us to measure the local out-of-plane

electric field components and to reveal the exact nature of the excited localized surface plasmon resonances.

Furthermore, the coupling mechanism between subsequent metal nanoparticles along the chain is experimentally

analyzed by spatial Fourier transformation on the complex near-field cartography, giving a direct experimental proof

of plasmonic Bloch mode propagation along array of localized surface plasmons. Our work demonstrates the possibility

to characterize multielement plasmonic nanostructures coupled to a photonic waveguide with a spatial resolution of

less than 30 nm. This experimental work constitutes a prerequisite for the development of integrated nanophotonic

devices.

http://pubs.acs.org/doi/abs/10.1021/nl304164y

Spin-dependent trapping of electrons at spinterfaces

Sabine Steil, Nicolas Großmann, Martin Laux, Andreas Ruffing, Daniel Steil, Martin Wiesenmayer, Stefan

Mathias,Oliver L. A. Monti, Mirko Cinchetti, Martin Aeschlimann

Hybrid ferromagnetic metal/organic interfaces—also known as spinterfaces—can exhibit highly efficient spin-filtering

properties and therefore present a promising class of materials for the future development of new spintronic

devices. Advancing the field depends critically on elucidating the fundamental microscopic processes that eventually

determine the spin-filtering properties in such hybrid structures. Here, we study the femtosecond spin dynamics at

the prototypical interface between cobalt and the metalorganic complex tris(8-hydroxyquinolinato)aluminium. To

disentangle the microscopic origin of spin filtering, we optically generate a transient spin polarization in a

well-defined hybrid interface state that we follow with a spin-resolved real-time pump–probe two-photon

photoemission experiment. We find that the electrons are trapped at the interface in a spin-dependent manner for a

surprisingly long time of the order of 0.5–1 ps. We conclude that ferromagnetic metal/organic interfaces act as spin

filters because electrons are trapped in hybrid interface states by spin-dependent confining potentials.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2548.html

Bipolar spin blockade and coherent state superpositions in a triple quantum dot

M. Busl, G. Granger, L. Gaudreau, R. Sánchez, A. Kam, M. Pioro-Ladrière, S. A. Studenikin, P. Zawadzki, Z. R. Wasilewski, A. S. Sachrajda & G. Platero

Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully1, 2. Readout of

the qubit state involves a conversion of spin to charge information, which is universally achieved by taking

advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade

transport signature in double quantum dots takes the form of a rectified current3. At present, more complex spin

qubit circuits including triple quantum dots are being developed4. Here we show, both experimentally and

theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar5 with current strongly

suppressed in both bias directions and also that a new quantum coherent mechanism becomes relevant. In this

mechanism, charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit

to the other, without involving the centre site. Our results have implications for future complex nanospintronic

circuits.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.7.html

Febr. 8- Febr. 15. (2013)

Válogatta: Fülöp Gergő

Electrical control over single hole spins in nanowire quantum dots

Single electron spins in semiconductor quantum dots (QDs) are a versatile platform for quantum information processing, however controlling decoherence remains a considerable challenge. Recently, hole spins have emerged as a promising alternative. Holes in III-V semiconductors have unique properties, such as strong spin-orbit interaction and weak coupling to nuclear spins, and therefore have potential for enhanced spin control and longer coherence times. Weaker hyperfine interaction has already been reported in self-assembled quantum dots using quantum optics techniques. However, challenging fabrication has so far kept the promise of hole-spin-based electronic devices out of reach in conventional III-V heterostructures. Here, we report gate-tuneable hole quantum dots formed in InSb nanowires. Using these devices we demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tuneable between hole and electron QDs, enabling direct comparison between the hyperfine interaction strengths, g-factors and spin blockade anisotropies in the two regimes.

http://arxiv.org/abs/1302.2648

Beating Classical Computing Without a Quantum Computer

http://www.sciencemag.org/content/339/6121/767.summary

Russian meteor largest in a century

http://www.nature.com/news/russian-meteor-largest-in-a-century-1.12438

Quantum engineering: Diamond envy

Nitrogen atoms trapped tens of nanometres apart in diamond can now be linked by quantum entanglement. This ability to produce and control entanglement in solid systems could enable powerful quantum computers.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2563.html

Room-temperature entanglement between single defect spins in diamond

Entanglement is the central yet fleeting phenomenon of quantum physics. Once being considered a peculiar counter-intuitive property of quantum theory1, it has developed into the most central element of quantum technology. Consequently, there have been a number of experimental demonstrations of entanglement between photons2, atoms3, ions4 and solid-state systems such as spins or quantum dots5, 6, 7, superconducting circuits8, 9 and macroscopic diamond10. Here we experimentally demonstrate entanglement between two engineered single solid-state spin quantum bits (qubits) at ambient conditions. Photon emission of defect pairs reveals ground-state spin correlation. Entanglement (fidelity = 0.67±0.04) is proved by quantum state tomography. Moreover, the lifetime of electron spin entanglement is extended to milliseconds by entanglement swapping to nuclear spins. The experiments mark an important step towards a scalable room-temperature quantum device being of potential use in quantum information processing as well as metrology.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2545.html

Three-dimensional optical manipulation of a single electron spin

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.259.html

Highly Transparent and Flexible Nanopaper Transistors

http://pubs.acs.org/doi/abs/10.1021/nn304407r

Spin-resolved Andreev levels in hybrid superconductor-semiconductor nanostructures

http://arxiv.org/pdf/1302.2611v1.pdf

Nov. 16 - Nov. 22 (2012)

Válogatta: Gubicza Ági

Memcomputing: a computing paradigm to store and process information on the same physical platform

M. Di Ventra, Y. V. Pershin

In present day technology, storing and processing of information occur on physically distinct regions of space. Not only does this result in space limitations; it also translates into unwanted delays in retrieving and processing of relevant information. There is, however, a class of two-terminal passive circuit elements with memory, memristive, memcapacitive and meminductive systems -- collectively called memelements -- that perform both information processing and storing of the initial, intermediate and final computational data on the same physical platform. Importantly, the states of these memelements adjust to input signals and provide analog capabilities unavailable in standard circuit elements, resulting in adaptive circuitry, and providing analog massively-parallel computation. All these features are tantalizingly similar to those encountered in the biological realm, thus offering new opportunities for biologically-inspired computation. Of particular importance is the fact that these memelements emerge naturally in nanoscale systems, and are therefore a consequence and a natural by-product of the continued miniaturization of electronic devices. We will discuss the various possibilities offered by memcomputing, discuss the criteria that need to be satisfied to realize this paradigm, and provide an example showing the solution of the shortest-path problem and demonstrate the healing property of the solution path.

http://xxx.lanl.gov/abs/1211.4487

Physics: Make nanotechnology research open-source

Joshua M. Pearce

To drive innovation at the nanoscale, the patent thicket must be chopped down, argues Joshua M. Pearce.

Any innovator wishing to work on or sell products based on single-walled carbon nanotubes in the United States must wade through more than 1,600 US patents that mention them1. He or she must obtain a fistful of licences just to use this tubular form of naturally occurring graphite rolled from a one-atom-thick sheet. This is because many patents lay broad claims: one nanotube example covers “a composition of matter comprising at least about 99% by weight of single-wall carbon molecules”. Tens of others make overlapping claims. ...(az absztrakt hosszan folytatódik a honlapon)

http://www.nature.com/nature/journal/v491/n7425/full/491519a.html

A wide-bandgap metal–semiconductor–metal nanostructure made entirely from graphene

J. Hicks, A. Tejeda, A. Taleb-Ibrahimi, M. S. Nevius, F. Wang, K. Shepperd, J. Palmer, F. Bertran, P. Le Fèvre, J. Kunc, W. A. de Heer,C. Berger & E. H. Conrad

Present methods for producing semiconducting–metallic graphene networks suffer from stringent lithographic demands, process-induced disorder in the graphene, and scalability issues. Here we demonstrate a one-dimensional metallic–semiconducting–metallic junction made entirely from graphene. Our technique takes advantage of the inherent, atomically ordered, substrate–graphene interaction when graphene is grown on SiC, in this case patterned SiC steps, and does not rely on chemical functionalization or finite-size patterning. This scalable bottom-up approach allows us to produce a semiconducting graphene strip whose width is precisely defined to within a few graphene lattice constants, a level of precision beyond modern lithographic limits, and which is robust enough that there is little variation in the electronic band structure across thousands of ribbons. The semiconducting graphene has a topographically defined few-nanometre-wide region with an energy gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2487.html

Discovery of the First True Three-Dimensional Topological Insulator: Samarium Hexaboride

Steven Wolgast, Cagliyan Kurdak, Kai Sun, J. W. Allen, Dae-Jeong Kim, Zachary Fisk

One of the most important new concepts in modern condensed matter physics is that there are new states of matter where the bulk is an unusual insulator in that it has topologically-protected metallic states on the surface. Although many important breakthroughs in the study of these surface states have been achieved within the last a few years, a very important link still remains missing -- the experimental discovery of a true 3D topological insulator. By definition, a topological insulator must have an insulating bulk and a topologically-protected metallic surface. However, among all the known 3D topological insulators (e.g. Bi$_{1-x}$Sb_x, Bi_2Se_3 and Bi_2Te_3), their bulk has until now found to be a conductor in which the topological index is not well-defined. In other words, these materials are not really topological insulators. Meanwhile, in the seemingly unrelated system of heavy-fermion insulators known as Kondo insulators, some long-standing puzzles have remained unanswered for over thirty years. In particular, it has been found that the strange transport results for some of the Kondo insulators cannot be understood if one assumes that the transport is governed by the 3D bulk. Here we study the transport properties of the heavy-fermion Kondo insulator SmB_6 with a novel configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that this material is a true topological insulator with an insulating bulk and a metallic surface. This discovery also resolves the last remaining puzzles about the strange transport behavior of this material.

http://xxx.lanl.gov/abs/1211.5104

Spin-Cavity Computing

Jelena Stajic

Today's (very rudimentary) quantum computers come in different guises, each with their own set of pros and cons. A growing trend has been to put two different technologies together in hybrid architectures in order to have the best of both worlds. One of the aims of such initiatives is to realize long-distance coupling between spin qubits (quantum bits based on electron spins) in quantum dots (zero-dimensional semiconductor nanostructures that allow controlled coupling of one or more electrons) via interactions with a superconducting microwave cavity. Petersson et al. made progress toward that goal by coupling a double quantum dot in an InSb nanowire to the electric field of a cavity, taking advantage of the strong spin-orbit interaction of InSb. The coupling was demonstrated by using a pulse sequence to electrically control the spin state, which was then read out in the phase response of the cavity. The estimated spin-cavity coupling is still shy of the strong limit required for two distant spin qubits to communicate; however, it is expected that improving the quality of the cavity and the coherence of the qubit, and/or using a material with stronger spin-orbit interactions, will bring physicists closer to that goal.

Nature 490, 380 (2012).

Thinning Segregated Graphene Layers on High Carbon Solubility Substrates of Rhodium Foils by Tuning the Quenching Process

Mengxi Liu, Yanfeng Zhang, Yubin Chen, Yabo Gao, Teng Gao, Donglin Ma, Qingqing Ji, Yu Zhang, Cong Li, and Zhongfan Liu

We report the synthesis of large-scale uniform graphene films on high carbon solubility substrates of Rh foils for the first time using an ambient-pressure chemical vapor deposition method. We find that, by increasing the cooling rate in the growth process, the thickness of graphene can be tuned from multilayer to monolayer, resulting from the different segregation amount of carbon atoms from bulk to surface. The growth feature was characterized with scanning electron microscopy, Raman spectra, transmission electron microscopy, and scanning tunneling microscopy. We also find that bilayer or few-layer graphene prefers to stack deviating from the Bernal stacking geometry, with the formation of versatile moiré patterns. On the basis of these results, we put forward a segregation growth mechanism for graphene growth on Rh foils. Of particular importance, we propose that this randomly stacked few-layer graphene can be a model system for exploring some fantastic physical properties such as van Hove singularities.

http://pubs.acs.org/doi/abs/10.1021/nn3047154

Epitaxial Graphene on 4H-SiC(0001) Grown under Nitrogen Flux: Evidence of Low Nitrogen Doping and High Charge Transfer

Emilio Velez-Fort, Claire Mathieu, Emiliano Pallecchi, Marine Pigneur, Mathieu G. Silly, Rachid Belkhou, Massimiliano Marangolo, Abhay Shukla, Fausto Sirotti, and Abdelkarim Ouerghi

Nitrogen doping of graphene is of great interest for both fundamental research to explore the effect of dopants on a 2D electrical conductor and applications such as lithium storage, composites, and nanoelectronic devices. Here, we report on the modifications of the electronic properties of epitaxial graphene thanks to the introduction, during the growth, of nitrogen-atom substitution in the carbon honeycomb lattice. High-resolution transmission microscopy and low-energy electron microscopy investigations indicate that the nitrogen-doped graphene is uniform at large scale. The substitution of nitrogen atoms in the graphene planes was confirmed by high-resolution X-ray photoelectron spectroscopy, which reveals several atomic configurations for the nitrogen atoms: graphitic-like, pyridine-like, and pyrrolic-like. Angle-resolved photoemission measurements show that the N-doped graphene exhibits large n-type carrier concentrations of 2.6 × 1013 cm–2, about 4 times more than what is found for pristine graphene, grown under similar pressure conditions. Our experiments demonstrate that a small amount of dopants (<1%) can significantly tune the electronic properties of graphene by shifting the Dirac cone about 0.3 eV toward higher binding energies with respect to the π band of pristine graphene, which is a key feature for envisioning applications in nanoelectronics.

http://pubs.acs.org/doi/abs/10.1021/nn304315z

Dielectric function, screening, and plasmons of graphene in the presence of spin-orbit interactions

Andreas Scholz, Tobias Stauber, and John Schlieman

We study the dielectric properties of graphene in the presence of Rashba and intrinsic spin-orbit interactions in their most general form, i.e., for arbitrary frequency, wave vector, doping, and spin-orbit coupling (SOC) parameters. The main result consists in the derivation of closed analytical expressions for the imaginary as well as for the real part of the polarization function. Several limiting cases, e.g., the case of purely Rashba or purely intrinsic SOC, and the case of equally large Rashba and intrinsic coupling parameters are discussed. In the static limit the asymptotic behavior of the screened potential due to charged impurities is derived. In the opposite limit (q=0, ω→0), an analytical expression for the plasmon dispersion is obtained and afterwards compared to the numerical result. Our result can also be applied to related systems such as bilayer graphene or topological insulators.

http://link.aps.org/doi/10.1103/PhysRevB.86.195424

Edge state effects in junctions with graphene electrodes

Dmitry A. Ryndyk, Jan Bundesmann, Ming-Hao Liu, and Klaus Richter

We consider plane junctions with graphene electrodes, which are formed by a single-level system (“molecule”) placed between the edges of two single-layer graphene half planes. We calculate the edge Green functions of the electrodes and the corresponding lead self-energies for the molecular levels in the cases of semi-infinite single-layer electrodes with armchair and zigzag edges. We show two main effects: first, a peculiar energy-dependent level broadening, reflecting at low energies the linear energy dependence of the bulk density of states in graphene, and, second, the shift and splitting of the molecular level energy, especially pronounced in the case of the zigzag edges due to the influence of the edge states. These effects give rise to peculiar conductance features at finite bias and gate voltages.

http://link.aps.org/doi/10.1103/PhysRevB.86.195425

Nov. 1 - Nov. 8 (2012)

Válogatta: Csontos Miklós

The fractional a.c. Josephson effect in a semiconductor–superconductor nanowire as a signature of Majorana particles

Leonid P. Rokhinson, Xinyu Liu and Jacek K. Furdyna

Topological superconductors that support Majorana fermions have been predicted when one-dimensional semiconducting wires are coupled to a superconductor. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor–superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radiofrequency f0 in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height ΔV = hf0/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge- 2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf0/e, suggesting that the supercurrent is carried by charge- e quasiparticles. This is a unique signature of the Majorana fermions, predicted almost 80 years ago.

http://www.nature.com/nphys/journal/v8/n11/pdf/nphys2429.pdf

Spatially resolved Hall effect measurement in a single semiconductor nanowire

Kristian Storm, Filip Halvardsson, Magnus Heurlin, David Lindgren, Anders Gustafsson, Phillip M. Wu, Bo Monemar and Lars Samuelson

Efficient light-emitting diodes and photovoltaic energy-harvesting devices are expected to play an important role in the continued efforts towards sustainable global power consumption. Semiconductor nanowires are promising candidates as the active components of both light-emitting diodes and photovoltaic cells, primarily due to the added freedom in device design offered by the nanowire geometry. However, for nanowire-based components to move past the proof-of-concept stage and be implemented in production-grade devices, it is necessary to precisely quantify and control fundamental material properties such as doping and carrier mobility. Unfortunately, the nanoscale geometry that makes nanowires interesting for applications also makes them inherently difficult to characterize. Here, we report a method to carry out Hall measurements on single core–shell nanowires. Our technique allows spatially resolved and quantitative determination of the carrier concentration and mobility of the nanowire shell. As Hall measurements have previously been completely unavailable for nanowires, the experimental platform presented here should facilitate the implementation of nanowires in advanced practical devices.

http://www.nature.com/nnano/journal/v7/n11/pdf/nnano.2012.190.pdf

Direct Observation of Valley Hybridization and Universal Symmetry of Graphene with Mesoscopic Conductance Fluctuations

Atindra Nath Pal, Vidya Kochat and Arindam Ghosh

In graphene, the valleys represent spinlike quantities and can act as a physical resource in valley-based electronics to produce novel quantum computation schemes. Here we demonstrate a direct route to tune and read the valley quantum states of disordered graphene by measuring the mesoscopic conductance fluctuations. We show that the conductance fluctuations in graphene at low temperatures are reduced by a factor of 4 when valley triplet states are gapped in the presence of short-range potential scatterers at high carrier densities. We also show that this implies a gate tunable universal symmetry class that outlines a fundamental feature arising from graphene’s unique crystal structure.

http://prl.aps.org/abstract/PRL/v109/i19/e196601

Unraveling Quantum Hall Breakdown in Bilayer Graphene with Scanning Gate Microscopy

M. R. Connolly, R. K. Puddy, D. Logoteta, P. Marconcini, M. Roy, J. P. Griffiths, G. A. C. Jones, P. A. Maksym, M. Macucci and C. G. Smith

Investigating the structure of quantized plateaus in the Hall conductance of graphene is a powerful way of probing its crystalline and electronic structure and will also help to establish whether graphene can be used as a robust standard of resistance for quantum metrology. We use low-temperature scanning gate microscopy to image the inter-plateau breakdown of the quantum Hall effect in an exfoliated bilayer graphene flake. Scanning gate images captured during breakdown exhibit intricate patterns where the conductance is strongly affected by the presence of the scanning probe tip. The maximum density and intensity of the tip-induced conductance perturbations occur at half-integer filling factors, midway between consecutive quantum Hall plateau, while the intensity of individual sites shows a strong dependence on tip-voltage. Our results are well-described by a model based on quantum percolation which relates the points of high responsivity to tip-induced scattering in a network of saddle points separating localized states.

http://pubs.acs.org/doi/pdf/10.1021/nl3015395

High Critical-Current Superconductor-InAs Nanowire-Superconductor Junctions

Simon Abay, Henrik Nilsson, Fan Wu, H.Q. Xu, C.M. Wilson and Per Delsing

We report on the fabrication of InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift-off nanofabrication method to get very short nanowire devices with Ohmic contacts. We observe very high critical currents of up to 800 nA in a wire with a diameter of 80 nm. The current−voltage characteristics of longer and suspended nanowires display either Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device.

http://pubs.acs.org/doi/pdf/10.1021/nl302740f

Ferroelectric Tunnel Memristor

D. J. Kim, H. Lu, S. Ryu, C.-W. Bark, C.-B. Eom, E. Y. Tsymbal and A. Gruverman

Strong interest in resistive switching phenomena is driven by a possibility to develop electronic devices with novel functional properties not available in conventional systems. Bistable resistive devices are characterized by two resistance states that can be switched by an external voltage. Recently, memristorselectric circuit elements with continuously tunable resistive behaviorhave emerged as a new paradigm for nonvolatile memories and adaptive electronic circuit elements. Employment of memristors can radically enhance the computational power and energy efficiency of electronic systems. Most of the existing memristor prototypes involve transition metal oxide resistive layers where conductive filaments formation and/or the interface contact resistance control the memristive behavior. In this paper, we demonstrate a new type of memristor that is based on a ferroelectric tunnel junction, where the tunneling conductance can be tuned in an analogous manner by several orders of magnitude by both the amplitude and the duration of the applied voltage. The ferroelectric tunnel memristors exhibit a reversible hysteretic nonvolatile resistive switching with a resistance ratio of up to 105 % at room temperature. The observed memristive behavior is attributed to the field-induced charge redistribution at the ferroelectric/electrode interface, resulting in the modulation of the interface barrier height.

http://pubs.acs.org/doi/pdf/10.1021/nl302912t

Programmable Sub-nanometer Sculpting of Graphene with Electron Beams

Felix Börrnert, Lei Fu, Sandeep Gorantla, Martin Knupfer, Bernd Büchner and Mark H. Rümmeli

Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without Cs correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains.

http://pubs.acs.org/doi/pdf/10.1021/nn304256a

Okt. 14 - Okt. 21 (2012)

Válogatta: Fülöp Gergő

Self-Assembled Colloidal Superparticles from Nanorods

Tie Wang1, Jiaqi Zhuang1, Jared Lynch1, Ou Chen1, Zhongliang Wang1, Xirui Wang1, Derek LaMontagne1, Huimeng Wu1, Zhongwu Wang2, Y. Charles Cao1,*

Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide–cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.

http://www.sciencemag.org/content/338/6105/358.abstract

Voltage-dependent conductance of a single graphene nanoribbon

Matthias Koch, Francisco Ample, Christian Joachim & Leonhard Grill

Graphene nanoribbons could potentially be used to create molecular wires with tailored conductance properties. However, understanding charge transport through a single molecule requires length-dependent conductance measurements and a systematic variation of the electrode potentials relative to the electronic states of the molecule1, 2. Here, we show that the conductance properties of a single molecule can be correlated with its electronic states. Using a scanning tunnelling microscope, the electronic structure of a long and narrow graphene nanoribbon, which is adsorbed on a Au(111) surface, is spatially mapped and its conductance then measured by lifting the molecule off the surface with the tip of the microscope. The tunnelling decay length is measured over a wide range of bias voltages, from the localized Tamm states over the gap up to the delocalized occupied and unoccupied electronic states of the nanoribbon. We also show how the conductance depends on the precise atomic structure and bending of the molecule in the junction, illustrating the importance of the edge states and a planar geometry.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.169.html

Unraveling Quantum Hall Breakdown in Bilayer Graphene with Scanning Gate Microscopy

M. R. Connolly *†‡, R. K. Puddy †, D. Logoteta §, P. Marconcini §, M. Roy , J. P. Griffiths †, G. A. C. Jones †, P. A. Maksym , M. Macucci §, and C. G. Smith †

Investigating the structure of quantized plateaus in the Hall conductance of graphene is a powerful way of probing its crystalline and electronic structure and will also help to establish whether graphene can be used as a robust standard of resistance for quantum metrology. We use low-temperature scanning gate microscopy to image the interplateau breakdown of the quantum Hall effect in an exfoliated bilayer graphene flake. Scanning gate images captured during breakdown exhibit intricate patterns where the conductance is strongly affected by the presence of the scanning probe tip. The maximum density and intensity of the tip-induced conductance perturbations occur at half-integer filling factors, midway between consecutive quantum Hall plateau, while the intensity of individual sites shows a strong dependence on tip-voltage. Our results are well-described by a model based on quantum percolation which relates the points of high responsivity to tip-induced scattering in a network of saddle points separating localized states.

http://pubs.acs.org/doi/full/10.1021/nl3015395

In Situ Observation of Electrostatic and Thermal Manipulation of Suspended Graphene Membranes

Wenzhong Bao †, Kevin Myhro †, Zeng Zhao †, Zhen Chen ‡, Wanyoung Jang ‡, Lei Jing †, Feng Miao †, Hang Zhang †, Chris Dames ‡, and Chun Ning Lau *†

Graphene is nature’s thinnest elastic membrane, and its morphology has important impacts on its electrical, mechanical, and electromechanical properties. Here we report manipulation of the morphology of suspended graphene via electrostatic and thermal control. By measuring the out-of-plane deflection as a function of applied gate voltage and number of layers, we show that graphene adopts a parabolic profile at large gate voltages with inhomogeneous distribution of charge density and strain. Unclamped graphene sheets slide into the trench under tension; for doubly clamped devices, the results are well-accounted for by membrane deflection with effective Young’s modulus E = 1.1 TPa. Upon cooling to 100 K, we observe buckling-induced ripples in the central portion and large upward buckling of the free edges, which arises from graphene’s large negative thermal expansion coefficient.

http://pubs.acs.org/doi/abs/10.1021/nl301836q

Quantitative Atomic Resolution Force Imaging on Epitaxial Graphene with Reactive and Nonreactive AFM Probes

Mark P. Boneschanscher †, Joost van der Lit †, Zhixiang Sun †‡, Ingmar Swart †, Peter Liljeroth §*, and Daniël Vanmaekelbergh †*

Atomic force microscopy (AFM) images of graphene and graphite show contrast with atomic periodicity. However, the contrast patterns vary depending on the atomic termination of the AFM tip apex and the tip–sample distance, hampering the identification of the atomic positions. Here, we report quantitative AFM imaging of epitaxial graphene using inert (carbon-monoxide-terminated) and reactive (iridium-terminated) tips. The atomic image contrast is markedly different with these tip terminations. With a reactive tip, we observe an inversion from attractive to repulsive atomic contrast with decreasing tip–sample distance, while a nonreactive tip only yields repulsive atomic contrast. We are able to identify the atoms with both tips at any tip–sample distance. This is a prerequisite for future structural and chemical analysis of adatoms, defects, and the edges of graphene nanostructures, crucial for understanding nanoscale graphene devices.

http://pubs.acs.org/doi/abs/10.1021/nn3040155

Radio Frequency Charge Parity Meter M. D. Schroer1,*, M. Jung1, K. D. Petersson1, and J. R. Petta1,2

We demonstrate a total charge parity measurement by detecting the radio frequency signal that is reflected by a lumped-element resonator coupled to a single InAs nanowire double quantum dot. The high frequency response of the circuit is used to probe the effects of the Pauli exclusion principle at interdot charge transitions. Even parity charge transitions show a striking magnetic field dependence that is due to a singlet-triplet transition, while odd parity transitions are relatively insensitive to a magnetic field. The measured response agrees well with cavity input-output theory, allowing accurate measurements of the interdot tunnel coupling and the resonator-charge coupling rate gc/2π∼17 MHz.

http://prl.aps.org/abstract/PRL/v109/i16/e166804

Majorana Single-Charge Transistor

R. Hützen1, A. Zazunov1, B. Braunecker2, A. Levy Yeyati2, and R. Egger1

http://prl.aps.org/abstract/PRL/v109/i16/e166403

Full First-Principles Theory of Spin Relaxation in Group-IV Materials

O. D. Restrepo and W. Windl

http://prl.aps.org/abstract/PRL/v109/i16/e166604

Spin Hall effect in graphene due to random Rashba field A. Dyrdał1 and J. Barnaś1,2

http://prb.aps.org/abstract/PRB/v86/i16/e161401

Conductivity of suspended graphene at the Dirac point

I. V. Gornyi1,2, V. Yu. Kachorovskii1,2,3, and A. D. Mirlin

http://prb.aps.org/abstract/PRB/v86/i16/e165413

Stability of Majorana fermions in proximity-coupled topological insulator nanowires

A. M. Cook, M. M. Vazifeh, and M. Franz

http://prb.aps.org/abstract/PRB/v86/i15/e155431

Temperature-dependent electron mobility in InAs nanowires

http://xxx.lanl.gov/abs/1210.3665

Majorana fermions from Landau quantization in a superconductor--topological-insulator hybrid structure

http://xxx.lanl.gov/abs/1210.4057

Majorana modes and complex band structure of quantum wires

http://xxx.lanl.gov/abs/1210.4817

Cross-correlations mediated by Majorana bound states

http://xxx.lanl.gov/abs/1210.5050

A proposal to probe quantum non-locality of Majorana fermions in tunneling experiments

http://xxx.lanl.gov/abs/1210.5514

Entanglement of nanoelectromechanical oscillators by Cooper-pair tunneling

http://xxx.lanl.gov/abs/1210.0665

Feedback-controlled electromigration for the fabrication of point contacts

http://xxx.lanl.gov/abs/1210.1551

Okt 9 - Okt 13 (2012)

Válogatta: Scherübl Zoltán

Hall effect measurements on InAs nanowires

Ch. Blömers1, T. Grap1, M. I. Lepsa1, J. Moers1, St. Trellenkamp2, D. Grützmacher1, H. Lüth1, and Th. Schäpers1,3

We have processed Hall contacts on InAs nanowires grown by molecular beam epitaxy using an electron beam lithography process with an extremely high alignment accuracy. The carrier concentrations determined from the Hall effect measurements on these nanowires are lower by a factor of about 4 in comparison with those measured by the common field-effect technique. The results are used to evaluate quantitatively the charging effect of the interface and surface states.

http://apl.aip.org/resource/1/applab/v101/i15?&page=5

Geometry-induced reduction of the critical current in superconducting nanowires

D. Henrich1,*, P. Reichensperger1, M. Hofherr1, J. M. Meckbach1, K. Il'in1, M. Siegel1, A. Semenov2, A. Zotova3, and D. Yu. Vodolazov3

Reduction of the critical current in narrow superconducting NbN lines with sharp and rounded bends with respect to the critical current in straight lines was studied at different temperatures. We compare our experimental results with the reduction expected in the framework of the London model and the Ginsburg-Landau model. We have experimentally found that the reduction is significantly less than either model predicts. We also show that in our NbN lines the bends mostly contribute to the reduction of the critical current at temperatures well below the superconducting transition temperature.

http://prb.aps.org/abstract/PRB/v86/i14/e144504

Ferroelectric Tunnel Memristor

D. J. Kim †, H. Lu †, S. Ryu ‡, C.-W. Bark ‡, C.-B. Eom ‡, E. Y. Tsymbal †, and A. Gruverman *†

Strong interest in resistive switching phenomena is driven by a possibility to develop electronic devices with novel functional properties not available in conventional systems. Bistable resistive devices are characterized by two resistance states that can be switched by an external voltage. Recently, memristors—electric circuit elements with continuously tunable resistive behavior—have emerged as a new paradigm for nonvolatile memories and adaptive electronic circuit elements. Employment of memristors can radically enhance the computational power and energy efficiency of electronic systems. Most of the existing memristor prototypes involve transition metal oxide resistive layers where conductive filaments formation and/or the interface contact resistance control the memristive behavior. In this paper, we demonstrate a new type of memristor that is based on a ferroelectric tunnel junction, where the tunneling conductance can be tuned in an analogous manner by several orders of magnitude by both the amplitude and the duration of the applied voltage. The ferroelectric tunnel memristors exhibit a reversible hysteretic nonvolatile resistive switching with a resistance ratio of up to 105 % at room temperature. The observed memristive behavior is attributed to the field-induced charge redistribution at the ferroelectric/electrode interface, resulting in the modulation of the interface barrier height.

http://pubs.acs.org/doi/abs/10.1021/nl302912t

Transient Rayleigh Scattering: A New Probe of Picosecond Carrier Dynamics in a Single Semiconductor Nanowire

Mohammad Montazeri , Howard E. Jackson , and Leigh M. Smith * , Jan M. Yarrison-Rice , Jung-Hyun Kang , Qiang Gao , Hark Hoe Tan , and Chennupati Jagadish

Using a new technique, transient Rayleigh scattering, we show that measurements from a single GaAs/AlGaAs core–shell semiconductor nanowire provide sensitive and detailed information on the time evolution of the density and temperature of the electrons and holes after photoexcitation by an intense laser pulse. Through band filling, band gap renormalization, and plasma screening, the presence of a dense and hot electron–hole plasma directly influences the real and imaginary parts of the complex index of refraction that in turn affects the spectral dependence of the Rayleigh scattering cross-section in well-defined ways. By measuring this spectral dependence as a function of time, we directly determine the thermodynamically independent density and temperature of the electrons and holes as a function of time after pulsed excitation as the carriers thermalize to the lattice temperature. We successfully model the results by including ambipolar transport, recombination, and cooling through optic and acoustic phonon emission that quantify the hole mobility at 68,000 cm2/V·s, linear decay constant at 380 ps, bimolecular recombination rate at 4.8 × 10–9 cm3/s and the energy-loss rate of plasma due to optical and acoustic phonon emission.

http://pubs.acs.org/doi/abs/10.1021/nl302767u

High Critical-Current Superconductor-InAs Nanowire-Superconductor Junctions

Simon Abay †, Henrik Nilsson ‡, Fan Wu †, H.Q. Xu ‡§, C.M. Wilson †, and Per Delsing *†

We report on the fabrication of InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift-off nanofabrication method to get very short nanowire devices with Ohmic contacts. We observe very high critical currents of up to 800 nA in a wire with a diameter of 80 nm. The current–voltage characteristics of longer and suspended nanowires display either Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device.

http://pubs.acs.org/doi/abs/10.1021/nl302740f

A valley-spin qubit in a carbon nanotube

Edward A. Laird, Fei Pei, Leo. P. Kouwenhoven

Although electron spins in III-V semiconductor quantum dots have shown great promise as qubits, a major challenge is the unavoidable hyperfine decoherence in these materials. In group IV semiconductors, the dominant nuclear species are spinless, allowing for qubit coherence times that have been extended up to seconds in diamond and silicon. Carbon nanotubes are a particularly attractive host material, because the spin-orbit interaction with the valley degree of freedom allows for electrical manipulation of the qubit. In this work, we realise such a qubit in a nanotube double quantum dot. The qubit is encoded in two valley-spin states, with coherent manipulation via electrically driven spin resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by measuring the current in Pauli blockade. Arbitrary qubit rotations are demonstrated, and the coherence time is measured via Hahn echo. Although the measured decoherence time is o

http://arxiv.org/abs/1210.3085

Robust Quantum Gates for a Singlet-Triplet Spin Qubit

Hugo Ribeiro, J. R. Petta, Guido Burkard

We show that universal quantum control of a two-electron singlet-triplet spin qubit can be achieved using Landau-Zener-St\"uckelberg interferometry. Going beyond normal Landau-Zener dynamics with infinitely long constant velocity sweeps across an energy level anti-crossing, we focus on a physical system consisting of a two-electron double quantum dot, where the spin states can be admixtures of charge states and the level velocity can be tuned in a time-dependent fashion. Our results indicate that charge coherence must be treated on an equal footing with spin coherence. In particular, we predict the presence of finite-time effects, which result in population transfer even in cases of an incomplete sweep through the anti-crossing. The competing requirements of adiabaticity and coherence are reconciled using specially designed pulses with a tunable level velocity. As a relevant example, we demonstrate that a Hadamard gate can be implemented for a realistic set of conditions in a GaAs double quantum dot device.

http://arxiv.org/abs/1210.1957

Valley-spin blockade and spin resonance in carbon nanotubes

Fei Pei, Edward A. Laird, Gary A. Steele, Leo P. Kouwenhoven

Manipulation and readout of spin qubits in quantum dots made in III-V materials successfully rely on Pauli blockade that forbids transitions between spin-triplet and spin-singlet states. Quantum dots in group IV materials have the advantage of avoiding decoherence from the hyperfine interaction by purifying them with only zero-spin nuclei. Complications of group IV materials arise from the valley degeneracies in the electronic bandstructure. These lead to complicated multiplet states even for two-electron quantum dots thereby significantly weakening the selection rules for Pauli blockade. Only recently have spin qubits been realized in silicon devices where the valley degeneracy is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade can be observed by lifting valley degeneracy through disorder. In clean nanotubes, quantum dots have to be made ultra-small to obtain a large energy difference between the relevant multiplet states. Here we report on low-disorder nanotubes and demonstrate Pauli blockade based on both valley and spin selection rules. We exploit the bandgap of the nanotube to obtain a large level spacing and thereby a robust blockade. Single-electron spin resonance is detected using the blockade.

http://arxiv.org/abs/1210.2622

Okt 2. - Okt 10. (2012)

Válogatta: Scherübl Zoltán és Balogh Zoltán

Proximity-effect-induced superconducting phase in the topological insulator Bi2Se3

Fan Yang1, Fanming Qu1, Jie Shen1, Yue Ding1, Jun Chen1, Zhongqing Ji1, Guangtong Liu1, Jie Fan1, Changli Yang1, Liang Fu2, and Li Lu1,*

We have studied the electron transport properties of topological insulator-related material Bi2Se3 near the superconducting Pb-Bi2Se3 interface, and found that a superconducting state is induced over an extended volume in Bi2Se3. This state can carry a Josephson supercurrent, and demonstrates a gaplike structure in the conductance spectra as probed by a normal-metal electrode. The establishment of the gap is not by confining the electrons into a narrow space close to the superconductor-normal metal interface, as previously observed in other systems, but presumably via electron-electron attractive interaction in Bi2Se3.

http://prb.aps.org/abstract/PRB/v86/i13/e134504

Defect-Free <110> Zinc-Blende Structured InAs Nanowires Catalyzed by Palladium

Hongyi Xu , Yong Wang , Yanan Guo , Zhiming Liao , Qiang Gao , H. Hoe Tan , Chennupati Jagadish , and Jin Zou *

We report the epitaxial growth of defect-free zinc-blende structured InAs nanowires on GaAs{111}B substrates using palladium catalysts in a metalorganic chemical vapor deposition reactor. Through detailed morphological, structural, and chemical characterizations using electron microscopy, it is found that these defect-free InAs nanowires grew along the 1?1?0 directions with four low-energy {111} faceted side walls and {1?1?3?} nanowire/catalyst interfaces. It is anticipated that these defect-free 1?1?0 nanowires benefit from the fact that the nanowire/catalyst interfaces does not contain the {111} planes, and the nanowire growth direction is not along the 111 directions. This study provides an effective approach to control the crystal structure and quality of epitaxial IIIV nanowires.

http://pubs.acs.org/doi/abs/10.1021/nl303028u

Terahetz detection by heterostructed InAs/InSb nanowire based field effect transistors

A. Pitanti1, D. Coquillat2, D. Ercolani1, L. Sorba1, F. Teppe2, W. Knap2, G. De Simoni3, F. Beltram1,3, A. Tredicucci1, and M. S. Vitiello1

Heterostructured InAs/InSb nanowire (Nw) based field effect transistors (FET) have been fabricated and tested as Terahetz radiation detectors. While responsivity and noise equivalent power compare with the ones of InAs nanowire detectors, the presence of small-gap InSb semiconductor gives rise to interesting physical effects such an increase of the detected signal with charge injection through the wire, at odds with standard FET-detectors. Additionally, the photodetected signal voltage changes its sign after a threshold gate bias, which we explain considering surface-related transport and field asymmetries imposed by the use of a lateral gate electrode.

http://apl.aip.org/resource/1/applab/v101/i14/p141103_s1?bypassSSO=1

Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback

R. Vijay, C. Macklin, D. H. Slichter, S. J. Weber, K. W. Murch, R. Naik, A. N. Korotkov & I. Siddiqi

The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this ‘instantaneous’ process can be stretched using weak measurements1, 2, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback3, 4, 5, 6, 7, 8. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity9. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier10, 11, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely12. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements13, 14. Other applications include quantum state stabilization4, 7, 15, entanglement generation using measurement16, state purification17 and adaptive measurements18, 19.

http://www.nature.com/nature/journal/v490/n7418/full/nature11505.html

Charge sensing in a Si/SiGe quantum dot with a radio frequency superconducting single-electron transistor

Mingyun Yuan1, Zhen Yang1, D. E. Savage2, M. G. Lagally2, M. A. Eriksson2, and A. J. Rimberg1

We report the operation of a radio frequency superconducting single-electron transistor (rf-SSET) as a charge sensor for single and double Si/SiGe quantum dots (QDs). Real-time electron tunneling events are observed from the reflected signal of the rf-SSET with a charge sensitivity of 4×10−6 e/math, which demonstrates a fast charge detection time of a few tens of microseconds. Measurements of the reflected power are used to map out the stability diagram of the double quantum dot.

http://apl.aip.org/resource/1/applab/v101/i14/p142103_s1?bypassSSO=1

Static and dynamic study of magnetic properties in FeNi film on flexible substrate, effect of applied stresses

W. Karboul-Trojet, Y. Roussigné, D. Faurie and S. M. Chérif

In this paper, we present a study of magnetic properties of NiFe thin film deposited onto polymer substrate (Kapton®). A complete study of the magnetic anisotropy is made thanks to ferromagnetic resonance. In-plane and out-of-plane anisotropies in the film are obtained due to the elaboration process. Furthermore, the magnetization is manipulated by applying uniaxial stress in the film. The stress-induced apparition of stripes domains from a saturated configuration is evidenced by in situ Brillouin light scattering and magnetic force microscopy studies. The saturating field is derived from the Muller criterion. The magnetostriction coefficient is evaluated from the applied stress allowing the stripes domains regeneration.

http://www.springerlink.com/content/b480x14853076260/

Scanning probe microscopy: A discerning look at the bonds in a molecule

Owain Vaughan

The resolution of a scanning probe microscope can be improved by attaching a small molecule to the tip of a probe

and this has previously allowed individual atoms and bonds within adsorbed organic molecules to be resolved.

Researchers at IBM's Zurich Research Laboratory, Universidade de Santiago de Compostela and CEMES-CNRS…

http://www.nature.com/nnano/journal/v7/n10/full/nnano.2012.178.html#/access

Molecular electronics: Probing intramolecular circuit laws

Christian Joachim

Constructive quantum interference is verified experimentally in a parallel single-molecule circuit, potentially

offering an intuitive approach to designing intramolecular circuits.

http://www.nature.com/nnano/journal/v7/n10/full/nnano.2012.172.html

Single InAs/GaSb Nanowire Low-Power CMOS Inverter

Anil W. Dey, Johannes Svensson, B. Mattias Borg, Martin Ek , and Lars-Erik Wernersson

III–V semiconductors have so far predominately been employed for n-type transistors in high-frequency applications.

This development is based on the advantageous transport properties and the large variety of heterostructure

combinations in the family of III–V semiconductors. In contrast, reports on p-type devices with high hole mobility

suitable for complementary metal–oxide–semiconductor (CMOS) circuits for low-power operation are scarce. In

addition, the difficulty to integrate both n- and p-type devices on the same substrate without the use of complex

buffer layers has hampered the development of III–V based digital logic. Here, inverters fabricated from single

n-InAs/p-GaSb heterostructure nanowires are demonstrated in a simple processing scheme. Using undoped segments and

aggressively scaled high-κ dielectric, enhancement mode operation suitable for digital logic is obtained for both

types of transistors. State-of-the-art on- and off-state characteristics are obtained and the individual

long-channel n- and p-type transistors exhibit minimum subthreshold swings of SS = 98 mV/dec and SS = 400 mV/dec,

respectively, at Vds = 0.5 V. Inverter characteristics display a full signal swing and maximum gain of 10.5 with a

small device-to-device variability. Complete inversion is measured at low frequencies although large parasitic

capacitances deform the waveform at higher frequencies.

http://pubs.acs.org/doi/abs/10.1021/nl302658y

Spin-Flip Transitions Induced by Time-Dependent Electric Fields in Surfaces with Strong Spin-Orbit Interaction

Julen Ibañez-Azpiroz, Asier Eiguren, E. Ya. Sherman, and Aitor Bergara

We present a comprehensive theoretical investigation of the light absorption rate at a Pb/Ge(111)-β√3×√3R30° surface

with strong spin-orbit coupling. Our calculations show that electron spin-flip transitions cause as much as 6% of

the total light absorption, representing 1 order of magnitude enhancement over Rashba-like systems. Thus, we

demonstrate that a substantial part of the light irradiating this nominally nonmagnetic surface is attenuated in

spin-flip processes. Remarkably, the spin-flip transition probability is structured in well-defined hot spots within

the Brillouin zone, where the electron spin experiences a sudden 90° rotation. This mechanism offers the possibility

of an experimental approach to the spin-orbit phenomena by optical means.

http://prl.aps.org/abstract/PRL/v109/i15/e156401

Experimental verification of contact-size estimates in point-contact spectroscopy on superconductor/ferromagnet heterocontacts

J. Gramich, P. Brenner, C. S¨urgers, H. v. L¨ohneysen and G. Goll

Nanostructured superconductor/ferromagnet heterocontacts are studied in the different transport regimes of point-contact spectroscopy. Direct measurements of the nanocontact size by scanning electron microscopy allow a comparison with theoretical models for contact-size estimates of heterocontacts. Our experimental data give evidence that size estimates yield reasonable values for the point-contact diameter d as long as the samples are carefully characterized with respect to the local electronic parameters.

http://prb.aps.org/toc/PRB/v86/i16

Ab initio study of the thermopower of biphenyl-based single-molecule junctions

M. B¨urkle, L. A. Zotti, J. K. Viljas, D. Vonlanthen, A. Mishchenko, T. Wandlowski, M. Mayor

By employing ab initio electronic-structure calculations combined with the nonequilibrium Green’s function technique,we study the dependence of the thermopowerQon the conformation in biphenyl-based single-molecule junctions. For the series of experimentally available biphenyl molecules, alkyl side chains allow us to gradually adjust the torsion angle ϕ between the two phenyl rings from 0◦ to 90◦ and to control in this way the degree of π-electron conjugation. Studying different anchoring groups and binding positions, our theory predicts that the absolute values of the thermopower decrease slightly towards larger torsion angles, following an a + b cos2 ϕ dependence. The anchoring group determines the sign ofQand a,b simultaneously. Sulfur and amine groups give rise to Q,a,b > 0, while for cyano, Q,a,b < 0. The different binding positions can lead to substantial variations of the thermopower mostly due to changes in the alignment of the frontier molecular orbital levels and the Fermi energy. We explain our ab initio results in terms of a π-orbital tight-binding model and a minimal two-level model, which describes the pair of hybridizing frontier orbital states on the two phenyl rings. The variations of the thermopower with ϕ seem to be within experimental resolution.

http://prb.aps.org/abstract/PRB/v86/i11/e115304

Szept. 21.- szept. 28. (2012)

Válogatta: Magda Gábor

Valley–spin blockade and spin resonance in carbon nanotubes

Fei Pei, Edward A. Laird, Gary A. Steele & Leo P. Kouwenhoven

The manipulation and readout of spin qubits in quantum dots have been successfully achieved using Pauli blockade, which forbids transitions between spin–triplet and spin–singlet states. Compared with spin qubits realized in III–V materials, group IV materials such as silicon and carbon are attractive for this application because of their low decoherence rates (nuclei with zero spins). However, valley degeneracies in the electronic band structure of these materials combined with Coulomb interactions reduce the energy difference between the blocked and unblocked states, significantly weakening the selection rules for Pauli blockade. Recent demonstrations of spin qubits in silicon devices have required strain and spatial confinement to lift the valley degeneracy. In carbon nanotubes, Pauli blockade can be observed by lifting valley degeneracy through disorder, but this makes the confinement potential difficult to control. To achieve Pauli blockade in low-disorder nanotubes, quantum dots have to be made ultrasmall, which is incompatible with conventional fabrication methods. Here, we exploit the bandgap of low-disorder nanotubes to demonstrate robust Pauli blockade based on both valley and spin selection rules. We use a novel stamping technique to create a bent nanotube, in which single-electron spin resonance is detected using the blockade. Our results indicate the feasibility of valley–spin qubits in carbon nanotubes.

http://www.nature.com/nnano/journal/v7/n10/full/nnano.2012.160.html

Subtractive Patterning via Chemical Lift-Off Lithography

Wei-Ssu Liao, Sarawut Cheunkar, Huan H. Cao, Heidi R. Bednar, Paul S. Weiss, and Anne M. Andrews

Conventional soft-lithography methods involving the transfer of molecular “inks” from polymeric stamps to substrates often encounter micrometer-scale resolution limits due to diffusion of the transferred molecules during printing. We report a “subtractive” stamping process in which silicone rubber stamps, activated by oxygen plasma, selectively remove hydroxyl-terminated alkanethiols from self-assembled monolayers (SAMs) on gold surfaces with high pattern fidelity. The covalent interactions formed at the stamp-substrate interface are sufficiently strong to remove not only alkanethiol molecules but also gold atoms from the substrate. A variety of high-resolution patterned features were fabricated, and stamps were cleaned and reused many times without feature deterioration. The remaining SAM acted as a resist for etching exposed gold features. Monolayer backfilling into the lift-off areas enabled patterned protein capture, and 40-nanometer chemical patterns were achieved.

http://www.sciencemag.org/content/337/6101/1517.abstract

Quantum-Enhanced Optical-Phase Tracking

Hidehiro Yonezawa, Daisuke Nakane, Trevor A. Wheatley, Kohjiro Iwasawa, Shuntaro Takeda, Hajime Arao, Kentaro Ohki, Koji Tsumura, Dominic W. Berry, Timothy C. Ralph, Howard M. Wiseman, Elanor H. Huntington, and Akira Furusawa

Tracking a randomly varying optical phase is a key task in metrology, with applications in optical communication. The best precision for optical-phase tracking has until now been limited by the quantum vacuum fluctuations of coherent light. Here, we surpass this coherent-state limit by using a continuous-wave beam in a phase-squeezed quantum state. Unlike in previous squeezing-enhanced metrology, restricted to phases with very small variation, the best tracking precision (for a fixed light intensity) is achieved for a finite degree of squeezing because of Heisenberg’s uncertainty principle. By optimizing the squeezing, we track the phase with a mean square error 15 ± 4% below the coherent-state limit.

http://www.sciencemag.org/content/337/6101/1514.abstract

The fractional a.c. Josephson effect in a semiconductor–superconductor nanowire as a signature of Majorana particles

Leonid P. Rokhinson, Xinyu Liu & Jacek K. Furdyna

Topological superconductors that support Majorana fermions have been predicted when one-dimensional semiconducting wires are coupled to a superconductor. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor–superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radiofrequency f0 in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height ΔV = hf0/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge- 2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf0/e, suggesting that the supercurrent is carried by charge- e quasiparticles. This is a unique signature of the Majorana fermions, predicted almost 80 years ago.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2429.html

Super-resolution Fluorescence Quenching Microscopy of Graphene

Rainer J. Stöhr, Roman Kolesov, Kangwei Xia, Rolf Reuter, Jan Meijer, Gennady Logvenov, and Jörg Wrachtrup

Lately, fluorescence quenching microscopy (FQM) has been introduced as a new tool to visualize graphene-based sheets. Even though quenching of the emission from a dye molecule by fluorescence resonance energy transfer (FRET) to graphene happens on the nanometer scale, the resolution of FQM so far is still limited to several hundreds of nanometers due to the Abbe limit restricting the resolution of conventional light microscopy. In this work, we demonstrate an advancement of FQM by using a super-resolution imaging technique for detecting fluorescence of color centers used in FQM. The technique is similar to stimulated emission depletion microscopy (STED). The combined “FRET+STED” technique introduced here for the first time represents a substantial improvement to FQM since it exhibits in principle unlimited resolution while still using light in the visible spectral range. In the present case we demonstrate all-optical imaging of graphene with resolution below 30 nm. The performance of the technique in terms of imaging resolution and contrast is well described by a theoretical model taking into account the general distance dependence of the FRET process and the distance distribution of donor centers with respect to the flake. In addition, the change in lifetime for partially quenched emitters allows extracting the quenching distance from experimental data for the first time.

http://pubs.acs.org/doi/abs/10.1021/nn303510p

Plasmon-Induced Doping of Graphene

Zheyu Fang, Yumin Wang, Zheng Liu, Andrea Schlather, Pulickel M. Ajayan, Frank H. L. Koppens, Peter Nordlander, and Naomi J. Halas

A metallic nanoantenna, under resonant illumination, injects nonequilibrium hot electrons into a nearby graphene structure, effectively doping the material. A prominent change in carrier density was observed for a plasmonic antenna-patterned graphene sheet following laser excitation, shifting the Dirac point, as determined from the gate-controlled transport characteristic. The effect is due to hot electron generation resulting from the decay of the nanoantenna plasmon following resonant excitation. The effect is highly tunable, depending on the resonant frequency of the plasmonic antenna, as well as on the incident laser power. Hot electron-doped graphene represents a new type of hybrid material that shows great promise for optoelectronic device applications.

http://pubs.acs.org/doi/abs/10.1021/nn304028b

Szept. 6. - Szept. 20. (2012)

Válogatta: Magyarkuti András

Nanocarbon-Based Photovoltaics

Marco Bernardi †, Jessica Lohrman ‡, Priyank V. Kumar †, Alec Kirkeminde ‡, Nicola Ferralis †, Jeffrey C. Grossman †*, and Shenqiang Ren ‡

Carbon materials are excellent candidates for photovoltaic solar cells: they are Earth-abundant, possess high optical absorption, and maintain superior thermal and photostability. Here we report on solar cells with active layers made solely of carbon nanomaterials that present the same advantages of conjugated polymer-based solar cells, namely, solution processable, potentially flexible, and chemically tunable, but with increased photostability and the possibility to revert photodegradation. The device active layer composition is optimized using ab initio density functional theory calculations to predict type-II band alignment and Schottky barrier formation. The best device fabricated is composed of PC70BM fullerene, semiconducting single-walled carbon nanotubes, and reduced graphene oxide. This active-layer composition achieves a power conversion efficiency of 1.3%—a record for solar cells based on carbon as the active material—and we calculate efficiency limits of up to 13% for the devices fabricated in this work, comparable to those predicted for polymer solar cells employing PCBM as the acceptor. There is great promise for improving carbon-based solar cells considering the novelty of this type of device, the high photostability, and the availability of a large number of carbon materials with yet untapped potential for photovoltaics. Our results indicate a new strategy for efficient carbon-based, solution-processable, thin film, photostable solar cells.

http://pubs.acs.org/doi/full/10.1021/nn302893p

Smartly Aligning Nanowires by a Stretching Strategy and Their Application As Encoded Sensors

Yuchen Wu †, Bin Su †*, and Lei Jiang †‡

The nanotechnology world is being more and more attracted toward high aspect ratio one-dimensional nanostructures due to their potentials as building blocks for electronic/optical devices. Here, we propose a novel method to generate nanowire patterns with assistance of superhydrophobic flexible polydimethylsiloxane (PDMS) substrates. Micropillar gaps are tunable via a stretching process of the PDMS surface; thus, diverse nanowire patterns can be formed by stretching the same PDMS surface in various ways. Importantly, square nanowire loops with alternative compositions can be generated through a double-stretching process, showing an advanced methodology in controlling the alignment of nanowires. Since alternative fluorescent molecules will be quenched by diverse chemical substances, this alternative nanowire loop shows a selective detection for diverse target compounds, which greatly improves the application of this nanowire patterning approach. Furthermore, such alternative nanowire patterns can be transferred from pillar-structured surfaces to flat films, indicating further potentials in microcircuits, sensitive sensors, and other organic functional nanodevices.

http://pubs.acs.org/doi/full/10.1021/nn303098n

Toward the Synthesis of Wafer-Scale Single-Crystal Graphene on Copper Foils

Zheng Yan †, Jian Lin ‡§, Zhiwei Peng †, Zhengzong Sun †, Yu Zhu †, Lei Li †, Changsheng Xiang †, E. Loïc Samuel †, Carter Kittrell †‡, and James M. Tour †‡§*

In this research, we constructed a controlled chamber pressure CVD (CP-CVD) system to manipulate graphene’s domain sizes and shapes. Using this system, we synthesized large (4.5 mm2) single-crystal hexagonal monolayer graphene domains on commercial polycrystalline Cu foils (99.8% purity), indicating its potential feasibility on a large scale at low cost. The as-synthesized graphene had a mobility of positive charge carriers of 11 000 cm2 V–1 s–1 on a SiO2/Si substrate at room temperature, suggesting its comparable quality to that of exfoliated graphene. The growth mechanism of Cu-based graphene was explored by studying the influence of varied growth parameters on graphene domain sizes. Cu pretreatments, electrochemical polishing, and high-pressure annealing are shown to be critical for suppressing graphene nucleation site density. A pressure of 108 Torr was the optimal chamber pressure for the synthesis of large single-crystal monolayer graphene. The synthesis of one graphene seed was achieved on centimeter-sized Cu foils by optimizing the flow rate ratio of H2/CH4. This work should provide clear guidelines for the large-scale synthesis of wafer-scale single-crystal graphene, which is essential for the optimized graphene device fabrication.

http://pubs.acs.org/doi/full/10.1021/nn303352k

A Device for Performing Lateral Conductance Measurements on Individual Double-Stranded DNA Molecules

Laurent D. Menard , Chad E. Mair , Michael E. Woodson , Jean Pierre Alarie , and J. Michael Ramsey *

A nanofluidic device is described that is capable of electrically monitoring the driven translocation of DNA molecules through a nanochannel. This is achieved by intersecting a long transport channel with a shorter orthogonal nanochannel. The ionic conductance of this transverse nanochannel is monitored while DNA is electrokinetically driven through the transport channel. When DNA passes the intersection, the transverse conductance is altered, resulting in a transient current response. In 1 M KCl solutions, this was found to be a current enhancement of 5–25%, relative to the baseline transverse ionic current. Two different device geometries were investigated. In one device, the DNA was detected after it was fully inserted into and translocating through the transport nanochannel. In the other device, the DNA was detected while it was in the process of entering the nanochannel. It was found that these two conditions are characterized by different transport dynamics. Simultaneous optical and electrical monitoring of DNA translocation confirmed that the transient events originated from DNA transport through the nanochannel intersection.

http://pubs.acs.org/doi/full/10.1021/nn303322r

Photocontrolled Molecular Structural Transition and Doping in Graphene

Namphung Peimyoo †, Jiewei Li †‡, Jingzhi Shang †, Xiaonan Shen †, Caiyu Qiu †, Linghai Xie ‡, Wei Huang ‡, and Ting Yu *†§

We studied chemical doping of trans- and cis-azobenzene on graphene by Raman spectroscopy. It was found that the molecule induces hole-doping in graphene through charge transfer. Moreover, the doping level in graphene can be reversibly modulated by a photocontrolled molecular conformation change. As trans-azobenzene isomerizes to the cis configuration under UV irradiation, we probe the dynamic molecular structural evolution of azobenzene on graphene by Raman spectroscopy. Raman analysis indicates the precise orientation of cis-azobenzene on the graphene surface, which brings us further comprehension of the effect of conformation change on the electronic properties of graphene. In particular, the substantial decreases of the doping level and chemical enhancement of the molecular signal are attributed to the weakening of hole transfer from molecule to graphene, owing to the lifting of the electron-withdrawing group away from the graphene. Moreover, the calculation results exhibit the favorable configuration of cis-azobenzene, which is in good agreement with Raman spectroscopic analysis. Our results highlight an approach for employing graphene as a promising platform for probing molecular conformation transition at the submolecular level by Raman spectroscopy.

http://pubs.acs.org/doi/full/10.1021/nn302876w

Coplanar-Gate Transparent Graphene Transistors and Inverters on Plastic

Beom Joon Kim †, Seoung-Ki Lee ‡, Moon Sung Kang §, Jong-Hyun Ahn ‡*, and Jeong Ho Cho †*

Transparent flexible graphene transistors and inverters in a coplanar-gate configuration were presented for the first time using only two materials: graphene and an ion gel gate dielectric. The novel device configuration simplifies device fabrication such that only two printing steps were required to fabricate transistors and inverters. The devices exhibited excellent device performances including low-voltage operation with a high transistor-on-current and mobility, excellent mechanical flexibility, environmental stability, and a reasonable inverting behavior upon connecting the two transistors.

http://pubs.acs.org/doi/full/10.1021/nn3020486

Ultrasensitive Flexible Graphene Based Field-Effect Transistor (FET)-Type Bioelectronic Nose

Seon Joo Park †, Oh Seok Kwon †, Sang Hun Lee ‡, Hyun Seok Song ‡, Tai Hyun Park *‡, and Jyongsik Jang *†

Rapid and precise discrimination of various odorants is vital to fabricating enhanced sensing devices in the fields of disease diagnostics, food safety, and environmental monitoring. Here, we demonstrate an ultrasensitive and flexible field-effect transistor (FET) olfactory system, namely, a bioelectronic nose (B-nose), based on plasma-treated bilayer graphene conjugated with an olfactory receptor. The stable p- and n-type behaviors from modified bilayer graphene (MBLG) took place after controlled oxygen and ammonia plasma treatments. It was integrated with human olfactory receptors 2AG1 (hOR2AG1: OR), leading to the formation of the liquid-ion gated FET-type platform. ORs bind to the particular odorant amyl butyrate (AB), and their interactions are specific and selective. The B-noses behave as flexible and transparent sensing devices and can recognize a target odorant with single-carbon-atom resolution. The B-noses are ultrasensitive and highly selective toward AB. The minimum detection limit (MDL) is as low as 0.04 fM (10–15; signal-to-noise: 4.2), and the equilibrium constants of OR-oxygen plasma-treated graphene (OR-OG) and ammonia plasma-treated graphene (-NG) are ca. 3.44 × 1014 and 1.47 × 1014 M–1, respectively. Additionally, the B-noses have long-term stability and excellent mechanical bending durability in flexible systems.

http://pubs.acs.org/doi/full/10.1021/nl301714x

Correlated Magnetic States in Extended One-Dimensional Defects in Graphene

Simone S. Alexandre †, A. D. Lúcio ‡, A. H. Castro Neto §, and R. W. Nunes *†

Ab initio calculations indicate that while the electronic states introduced by tilt grain boundaries in graphene are only partially confined to the defect core, a translational grain boundary introduces states near the Fermi level that are very strongly confined to the core of the defect, and display a ferromagnetic instability. The translational boundary lies along a graphene zigzag direction and its magnetic state is akin to that which has been theoretically predicted to occur on zigzag edges of graphene ribbons. Unlike ribbon edges, the translational grain boundary is fully immersed within the bulk of graphene, hence its magnetic state is protected from the contamination and reconstruction effects that have hampered experimental detection of the magnetic ribbon states. Moreover, our calculations suggest that charge transfer between grain boundaries and the bulk in graphene is short ranged, with charge redistribution confined to 5 Å from the geometric center of the 1D defects.

http://pubs.acs.org/doi/full/10.1021/nl3017434

Understanding the Impact of Schottky Barriers on the Performance of Narrow Bandgap Nanowire Field Effect Transistors

Yanjie Zhao †, Drew Candebat ‡§, Collin Delker ‡§, Yunlong Zi †, David Janes ‡§, Joerg Appenzeller ‡§, and Chen Yang *†

Semiconductor nanowires have been explored as alternative electronic materials for high performance device applications exhibiting low power consumption specs. Electrical transport in III–V nanowire (NW) field-effect transistors (FETs) is frequently governed by Schottky barriers between the source/drain and the NW channel. Consequently the device performance is greatly impacted by the contacts. Here we present a simple model that explains how ambipolar device characteristics of NW-FETs and in particular the achievable on/off current ratio can be analyzed to gain a detailed idea of (a) the bandgap of the synthesized NWs and (b) the potential performance of various NW materials. In particular, we compare the model with our own transport measurements on InSb and InAs NW-FETs as well as results published by other groups. The analysis confirms excellent agreement with the predictions of the model, highlighting the potential of our approach to understand novel NW based materials and devices and to bridge material development and device applications.

http://pubs.acs.org/doi/full/10.1021/nl302684s

Quantum computation: Spinning towards scalable circuits

Lee C. Bassett & David D. Awschalom

Silicon devices form the backbone of modern computers. It turns out that they might also be a natural hardware platform for a new era of computing technology that uses the principles of quantum physics.

Nature (2012) doi:10.1038/nature11488 http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11488.html

A single-atom electron spin qubit in silicon

Jarryd J. Pla, Kuan Y. Tan, Juan P. Dehollain, Wee H. Lim, John J. L. Morton, David N. Jamieson, Andrew S. Dzurak & Andrea Morello

A single atom is the prototypical quantum system, and a natural candidate for a quantum bit, or qubit—the elementary unit of a quantum computer. Atoms have been successfully used to store and process quantum information in electromagnetic traps1, as well as in diamond through the use of the nitrogen–vacancy-centre point defect2. Solid-state electrical devices possess great potential to scale up such demonstrations from few-qubit control to larger-scale quantum processors. Coherent control of spin qubits has been achieved in lithographically defined double quantum dots in both GaAs (refs 3–5) and Si (ref. 6). However, it is a formidable challenge to combine the electrical measurement capabilities of engineered nanostructures with the benefits inherent in atomic spin qubits. Here we demonstrate the coherent manipulation of an individual electron spin qubit bound to a phosphorus donor atom in natural silicon, measured electrically via single-shot read-out7, 8, 9. We use electron spin resonance to drive Rabi oscillations, and a Hahn echo pulse sequence reveals a spin coherence time exceeding 200 µs. This time should be even longer in isotopically enriched 28Si samples10, 11. Combined with a device architecture12 that is compatible with modern integrated circuit technology, the electron spin of a single phosphorus atom in silicon should be an excellent platform on which to build a scalable quantum computer.

Nature (2012) doi:10.1038/nature11449 http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11449.html

Aug.30. - Szept.6. (2012)

2012. szeptember 13.

Asbóth János - Előadás: A Berry-fázistól a Chern-számig -- és mi köze van ennek a topologikus szigetelőkhöz

foliak, cikk1, cikk2

Tóvári Endre (JC)

Probing the conductance superposition law in single-molecule circuits with parallel paths

H. Vazquez, R. Skouta, S. Schneebeli, M. Kamenetska, R. Breslow, L. Venkataraman and M.S. Hybertsen

According to Kirchhoff's circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role1, as exemplified by the Aharonov–Bohm effect in metal rings2, 3. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring4, 5. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions6, 7, 8, 9, 10. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference.

http://www.nature.com/nnano/journal/v7/n10/full/nnano.2012.147.html

Adsorbate Transport on Graphene by Electromigration

Dmitry Solenov, and Kirill A. Velizhanin

Chemical functionalization of graphene holds promise for various applications ranging from nanoelectronics to catalysis, drug delivery, and nanoassembly. In many applications it is important to be able to transport adsorbates on graphene in real time. We propose to use electromigration to drive the adsorbate transport across the graphene sheet. To assess the efficiency of electromigration, we develop a tightbinding model of electromigration of an adsorbate on graphene and obtain simple analytical expressions for different contributions to the electromigration force. Using experimentally accessible parameters of realistic graphene-based devices as well as electronic structure theory calculations to parametrize the developed model, we argue that electromigration on graphene can be efficient. As an example, we show that the drift velocity of atomic oxygen covalently bound to graphene can reach ~1 cm=s.

http://prl.aps.org/abstract/PRL/v109/i9/e095504

Appearance of Flat Bands and Edge States in Boron-Carbon-Nitride Nanoribbons

Tomoaki Kaneko and Kikuo Harigayay

Presence of flat bands and edge states at the Fermi level in graphene nanoribbons with zigzag edges is one of the most interesting and attracting properties of nanocarbon materials but it is believed that they are quite fragile states and disappear when B and N atoms are doped at around the edges. In this paper, we theoretically investigate electronic and magnetic properties of boron-carbon-nitride (BCN) nanoribbons with zigzag edges where the outermost C atoms on the edges are alternately replaced with B and N atoms using the first principles calculations. We show that BCN nanoribbons have the flat bands and edge states at the Fermi level in both H_2 rich and poor environments. The flat bands are similar to those at graphene nanoribbons with zigzag edges, but the distributions of charge and spin densities are different between them. A tight binding model and the Hubbard model analysis show that the difference in the distribution of charge and spin densities is caused by the different site energies of B and N atoms compared with C atoms.

http://xxx.lanl.gov/abs/1209.0258

Coherent Two-Electron Spin Qubits in an Optically Active Pair of Coupled InGaAs Quantum Dots

K. M. Weiss, J. M. Elzerman, Y. L. Delley, J. Miguel-Sanchez, and A. Imamoğlu

In semiconductors, the T2* coherence time of a single confined spin is limited either by the fluctuating magnetic environment (via the hyperfine interaction), or by charge fluctuations (via the spin-orbit interaction). We demonstrate that both limitations can be overcome simultaneously by using two exchange-coupled electron spins that realize a single decoherence-avoiding qubit. Using coherent population trapping, we generate a coherent superposition of the singlet and triplet states of an optically active quantum dot molecule, and show that the corresponding T2* may exceed 200 ns.

http://prl.aps.org/abstract/PRL/v109/i10/e107401

Detection of spin polarization utilizing singlet and triplet states in a single-lead quantum dot

Tomohiro Otsuka, Yuuki Sugihara, Jun Yoneda, Shingo Katsumoto, and Seigo Tarucha

We propose and demonstrate a new method to probe local spin polarization in semiconductor microdevices at low and zero magnetic fields. By connecting a single-lead quantum dot to a semiconductor microdevice and monitoring electron tunneling into singlet and triplet states in the dot, we can detect the local spin polarization formed in the target device. We confirm the validity of this detection scheme utilizing spin-split quantum Hall edge states. We also observe nonzero local spin polarization at the device edge in low magnetic fields, which is not detectable with conventional macroscopic probes.

http://prb.aps.org/abstract/PRB/v86/i8/e081308

Edge state transport through disordered graphene nanoribbons in the quantum Hall regime

Fabian Duerr, Jeroen B. Oostinga, Charles Gould, Laurens W. Molenkamp

The presence of strong disorder in graphene nanoribbons yields low-mobility diffusive transport at high charge densities, whereas a transport gap occurs at low densities. Here, we investigate the longitudinal and transverse magnetoresistance of a narrow (60 nm) nanoribbon in a six-terminal Hall bar geometry. At B= 11 T, quantum Hall plateaux appear at $\sigma_{xy}=\pm2e^2/h$, $\pm6e^2/h$ and $\pm10e^2/h$, for which the Landau level spacing is larger than the Landau level broadening. Interestingly, the transport gap does not disappear in the quantum Hall regime, when the zero-energy Landau level is present at the charge neutrality point, implying that it cannot originate from a lateral confinement gap. At high charge densities, the longitudinal and Hall resistance exhibit reproducible fluctuations, which are most pronounced at the transition regions between Hall plateaux. Bias-dependent measurements strongly indicate that these fluctuations can be attributed to phase coherent scattering in the disordered ribbon.

http://xxx.lanl.gov/abs/1208.6429

Experimental Implementation of Encoded Logical Qubit Operations in a Perfect Quantum Error Correcting Code

Jingfu Zhang, Raymond Laflamme, and Dieter Suter

Large-scale universal quantum computing requires the implementation of quantum error correction (QEC). While the implementation of QEC has already been demonstrated for quantum memories, reliable quantum computing requires also the application of nontrivial logical gate operations to the encoded qubits. Here, we present examples of such operations by implementing, in addition to the identity operation, the NOT and the Hadamard gate to a logical qubit encoded in a five qubit system that allows correction of arbitrary single-qubit errors. We perform quantum process tomography of the encoded gate operations, demonstrate the successful correction of all possible single-qubit errors, and measure the fidelity of the encoded logical gate operations.

http://prl.aps.org/abstract/PRL/v109/i10/e100503

Gate voltage modulation of spin-Hall-torque-driven magnetic switching

Luqiao Liu, Chi-Feng Pai, D. C. Ralph, R. A. Buhrman

Two promising strategies for achieving efficient control of magnetization in future magnetic memory and non-volatile spin logic devices are spin transfer torque from spin polarized currents and voltage-controlled magnetic anisotropy (VCMA). Spin transfer torque is in widespread development as the write mechanism for next-generation magnetic memory, while VCMA offers the potential of even better energy performance due to smaller Ohmic losses. Here we introduce a 3-terminal magnetic tunnel junction (MTJ) device that combines both of these mechanisms to achieve new functionality: gate-voltage-modulated spin torque switching. This gating makes possible both more energy-efficient switching and also improved architectures for memory and logic applications, including a simple approach for making magnetic memories with a maximum-density cross-point geometry that does not require a control transistor for every MTJ.

http://xxx.lanl.gov/abs/1209.0962

How Close Can One Approach the Dirac Point in Graphene Experimentally?

Alexander S. Mayorov, Daniel C. Elias, Ivan S. Mukhin, Sergey V. Morozov, Leonid A. Ponomarenko, Kostya S. Novoselov, A. K. Geim, and Roman V. Gorbachev

The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 106 cm2 V–1 s–1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as ≈108 cm–2, allowing a neutral state with a few charge carriers per entire micrometer-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap.

http://pubs.acs.org/doi/abs/10.1021/nl301922d

Máj. 11. - Jun. 7. (2012)

Válogatta: Makk Péter

Transport Properties of a Single-Molecule Diode

Emanuel Lörtscher†*, Bernd Gotsmann†, Youngu Lee‡, Luping Yu§, Charles Rettner, and Heike Riel†

Charge transport through single diblock dipyrimidinyl diphenyl molecules consisting of a donor and acceptor moiety was measured in the low-bias regime and as a function of bias at different temperatures using the mechanically controllable break-junction technique. Conductance histograms acquired at 10 mV reveal two distinct peaks, separated by a factor of 1.5, representing the two orientations of the single molecule with respect to the applied bias. The current–voltage characteristics exhibit a temperature-independent rectification of up to a factor of 10 in the temperature range between 300 and 50 K with single-molecule currents of 45–70 nA at ±1.5 V. The current–voltage characteristics are discussed using a semiempirical model assuming a variable coupling of the molecular energy levels as well as a nonsymmetric voltage drop across the molecular junction, thus shifting the energy levels accordingly. The excellent agreement of the data with the proposed model suggests that the rectification originates from an asymmetric Coulomb blockade in combination with an electric-field-induced level shifting.

http://pubs.acs.org/doi/abs/10.1021/nn300438h

Integration of Hexagonal Boron Nitride with Quasi-freestanding Epitaxial Graphene: Toward Wafer-Scale High-Performance Devices

Michael S. Bresnehan,†,‡ Matthew J. Hollander,‡,§ Maxwell Wetherington,†,‡ Michael LaBella,‡ Kathleen A. Trumbull,‡ Randal Cavalero,‡ David W. Snyder,‡,^ and Joshua A. Robinson†,*

Hexagonal boron nitride (h-BN) is a promising dielectric material for graphene-based electronic devices. Here we investigate the potential of h-BN gate dielectrics, grown by chemical vapor deposition (CVD), for integration with quasi-freestanding epitaxial graphene (QFEG). We discuss the large scale growth of h-BN on copper foil via a catalytic thermal CVD process and the subsequent transfer of h-BN to a 75 mm QFEG wafer. X-ray photoelectron spectroscopy (XPS) measurements confirm the absence of h-BN/graphitic domains and indicate that the film is chemically stable throughout the transfer process, while Raman spectroscopy indicates a 42% relaxation of compressive stress following removal of the copper substrate and subsequent transfer of h-BN to QFEG. Despite stress-induced wrinkling observed in the films, Hall effect measurements show little degradation (<10%) in carrier mobility for h-BN coated QFEG. Temperature dependent Hall measurements indicate little contribution from remote surface optical phonon scattering and suggest that, compared to HfO2 based dielectrics, h-BN can be an excellent material for preserving electrical transport properties. Graphene transistors utilizing h-BN gates exhibit peak intrinsic cutoff frequencies >30 GHz (2.4× that of HfO2-based devices).

http://pubs.acs.org/doi/abs/10.1021/nn300996t

Terahertz and Infrared Spectroscopy of Gated Large-Area Graphene

Lei Ren†, Qi Zhang†, Jun Yao#, Zhengzong Sun‡, Ryosuke Kaneko§, Zheng Yan‡, Sébastien Nanot†, Zhong Jin‡, Iwao Kawayama§, Masayoshi Tonouchi§, James M. Tour‡, and Junichiro Kono*†¶

We have fabricated a centimeter-size single-layer graphene device with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10–10 000 cm–1), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods were able to effectively tune the Fermi energy, EF, which in turn modified the Drude-like intraband absorption in the terahertz as well as the “2EF onset” for interband absorption in the mid-infrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.

http://pubs.acs.org/doi/abs/10.1021/nl301496r

Patterning of graphene

Ji Feng , Wenbin Li , Xiaofeng Qian , Jingshan Qi , Liang Qi and Ju Li

Two-dimensional atomic sheets of carbon (graphene, graphane, etc.) are amenable to unique patterning schemes such as cutting, bending, folding and fusion that are predicted to lead to interesting properties. In this review, we present theoretical understanding and processing routes for patterning graphene and highlight potential applications. With more precise and scalable patterning, the prospects of integrating flat carbon (graphene) with curved carbon (nanotubes & half nanotubes) and programmable graphene folding are envisioned.

http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr30790a

Spin polarization of the quantum spin Hall edge states

Christoph Brüne, Andreas Roth, Hartmut Buhmann, Ewelina M. Hankiewicz, Laurens W. Molenkamp, Joseph Maciejko, Xiao-Liang Qi & Shou-Cheng Zhang

The prediction and experimental verification of the quantum spin Hall state marked the discovery of a new state of matter now known as topological insulators. Two-dimensional topological insulators exhibit the quantum spin-Hall effect, characterized by gapless spin-polarized counter-propagating edge channels. Whereas the helical character of these edge channels is now well established, experimental confirmation that the transport in the edge channels is spin polarized is still outstanding. We report experiments on nanostructures fabricated from HgTe quantum wells with an inverted band structure, in which a split gate technique allows us to combine both quantum spin Hall and metallic spin Hall transport in a single device. In these devices, the quantum spin Hall effect can be used as a spin current injector and detector for the metallic spin Hall effect, and vice versa, allowing for an all-electrical detection of spin polarization.

http://www.nature.com/nphys/journal/v8/n6/full/nphys2322.html

Mechanical Annealing of Metallic Electrodes at the Atomic Scale

C. Sabater1, C. Untiedt1, J. J. Palacios2, and M. J. Caturla1,*

The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately 5G0 in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which show how, after repeated indentations (mechanical annealing), the two metallic electrodes are shaped into tips of reproducible structure. These results provide a crucial insight into fundamental aspects relevant to nanotribology or scanning probe microscopies.

http://prl.aps.org/abstract/PRL/v108/i20/e205502

Máj. 4. - Máj. 11. (2012)

Válogatta: Csontos Miklós

Spectroscopy of non-local superconducting correlations in a double quantum dot

L. G. Herrmann, P. Burset, W. J. Herrera, F. Portier, P. Roche, C. Strunk, A. Levy Yeyati, T. Kontos

We investigate non-linear transport in a double quantum dot connected to two normal electrodes and a central superconducting finger. By this means, we perform a transport spectroscopy of such a system which implements a Cooper pair splitter. The non-linear conductance exhibits strong subgap features which can be associated with the coherence of the injected Cooper pairs. Our findings are well accounted for by the recently developed microscopic theory of Cooper pairs splitters made in SWNTs.

http://arxiv.org/abs/1205.1972

Microwave spectroscopy of a Cooper pair beam splitter

Audrey Cottet

This article discusses how to demonstrate the entanglement of the split Cooper pairs produced in a double-quantum-dot based Cooper pair beam splitter (CPS), by performing the microwave spectroscopy of the CPS. More precisely, one can study the DC current response of such a CPS to two on-phase microwave gate irradiations applied to the two CPS dots. Some of the current peaks caused by the microwaves show a strongly nonmonotonic variation with the amplitude of the irradiation applied individually to one dot. This effect is directly due to a subradiance property caused by the coherence of the split pairs. Using realistic parameters, one finds that this effect has a measurable amplitude.

http://arxiv.org/abs/1205.2252

Direct Observation of Interband Spin-Orbit Coupling in a Two-Dimensional Electron System

Hendrik Bentmann, Samir Abdelouahed, Mattia Mulazzi, Jurgen Henk, and Friedrich Reinert

We report the direct observation of interband spin-orbit (SO) coupling in a two-dimensional (2D) surface electron system, in addition to the anticipated Rashba spin splitting. Using angle-resolved photoemission experiments and first-principles calculations on Bi-Ag-Au heterostructures, we show that the effect strongly modifies the dispersion as well as the orbital and spin character of the 2D electronic states, thus giving rise to considerable deviations from the Rashba model. The strength of the interband SO coupling is tuned by the thickness of the thin film structures.

http://prl.aps.org/abstract/PRL/v108/i19/e196801

Distinguishing Spontaneous Quantum Hall States in Bilayer Graphene

Fan Zhang and A. H. MacDonald

Chirally stacked N-layer graphene with N > 2 is susceptible to a variety of distinct broken symmetry states in which each spin-valley flavor spontaneously transfers charge between layers. In mean-field theory, one of the likely candidate ground states for a neutral bilayer is the layer antiferromagnet that has opposite spin polarizations in opposite layers. In this Letter, we analyze how the layer antiferromagnet and other competing states are influenced by Zeeman fields that couple to spin and by interlayer electric fields that couple to layer pseudospin, and comment on the possibility of using Zeeman responses and edge state signatures to identify the character of the bilayer ground state experimentally.

http://prl.aps.org/abstract/PRL/v108/i18/e186804

Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum

Luqiao Liu, Chi-Feng Pai, Y. Li, H. W. Tseng, D. C. Ralph, R. A. Buhrman1

Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in b-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

http://www.sciencemag.org/content/336/6081/555.full

Ápr. 27. - Máj. 3. (2012)

Válogatta: Halbritter András

Spin–orbital separation in the quasi-one-dimensional Mott insulator Sr2CuO3

J. Schlappa,1, 2 K. Wohlfeld,3 K. J. Zhou,1, 9 M. Mourigal,4 M. W. Haverkort,5 V. N. Strocov,1 L. Hozoi,3 C. Monney,1 S. Nishimoto,3 S. Singh,6, 9 A. Revcolevschi,6 J.-S. Caux,7 L. Patthey,1, 8 H. M. Rønnow,4 J. van den Brink3 & T. Schmitt

When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital1. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom2. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago3. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone.

http://www.nature.com/nature/journal/v485/n7396/full/nature10974.html

Revealing the Angular Symmetry of Chemical Bonds by Atomic Force Microscopy

Joachim Welker, Franz J. Giessibl

We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.

http://www.sciencemag.org/content/336/6080/444.abstract

Emergence of superlattice Dirac points in graphene on hexagonal boron nitride

Matthew Yankowitz, Jiamin Xue, Daniel Cormode, Javier D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, Pablo Jarillo-Herrero, Philippe Jacquod & Brian J. LeRoy

The Schrödinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of bandgaps near points of the reciprocal lattice known as Brillouin zone boundaries1. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a bandgap and instead new Dirac points appear in the electronic structure of the material2, 3. Graphene on hexagonal boron nitride exhibits a rotation-dependent moiré pattern4, 5. Here, we show experimentally and theoretically that this moiré pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy determined by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. Furthermore, the local density of states near these Dirac cones exhibits hexagonal modulation due to the influence of the periodic potential.

http://www.nature.com/nphys/journal/v8/n5/abs/nphys2272.html

Experimental observation of the optical spin transfer torque

P. Němec, E. Rozkotová, N. Tesařová, F. Trojánek, E. De Ranieri, K. Olejník, J. Zemen, V. Novák, M. Cukr, P. Malý & T. Jungwirth

Spin transfer torque—the transfer of angular momentum from a spin-polarized current to a ferromagnet’s magnetization—has already found commercial application in memory devices, but the underlying physics is still not fully understood. Researchers now demonstrate the crucial role played by the polarization of the laser light that generates the current; a subtle effect only evident when isolated from other influences such as heating.

http://dx.doi.org/doi:10.1038/nphys2279

Nature Nanotechnology news and views: Nanoelectronics: Transistors arrive at the atomic limit

Gabriel P. Lansbergen

A single-atom transistor has been made by positioning a phosphorus atom between metallic electrodes, also made of phosphorus, on a silicon surface.

http://www.nature.com/nnano/journal/v7/n4/full/nnano.2012.23.html

original paper:

A single-atom transistor

Martin Fuechsle, Jill A. Miwa, Suddhasatta Mahapatra, Hoon Ryu, Sunhee Lee, Oliver Warschkow, Lloyd C. L. Hollenberg, Gerhard Klimeck & Michelle Y. Simmons

The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope1 can manipulate individual atoms2 and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces3, but the fabrication of working devices—such as transistors with extremely short gate lengths4, spin-based quantum computers5, 6, 7, 8 and solitary dopant optoelectronic devices9—requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy10

http://www.nature.com/nnano/journal/v7/n4/abs/nnano.2012.21.html

Signatures of Cooperative Effects and Transport Mechanisms in Conductance Histograms

Matthew G. Reuter*†§, Mark C. Hersam†‡, Tamar Seideman†, and Mark A. Ratner†

We present a computational investigation into the line shapes of peaks in conductance histograms, finding that they possess high information content. In particular, the histogram peak associated with conduction through a single molecule elucidates the electron transport mechanism and is generally well-described by beta distributions. A statistical analysis of the peak corresponding to conduction through two molecules reveals the presence of cooperative effects between the molecules and also provides insight into the underlying conduction channels. This work describes tools for extracting additional interpretations from experimental statistical data, helping us better understand electron transport processes.

http://pubs.acs.org/doi/abs/10.1021/nl204379j

Transport Properties of Graphene Nanoroads in Boron Nitride Sheets

Jeil Jung†, Zhenhua Qiao*†, Qian Niu†‡, and Allan H. MacDonald†

We demonstrate that the one-dimensional (1D) transport channels that appear in the gap when graphene nanoroads are embedded in boron nitride (BN) sheets are more robust when they are inserted at AB/BA grain boundaries. Our conclusions are based on ab initio electronic structure calculations for a variety of different crystal orientations and bonding arrangements at the BN/C interfaces. This property is related to the valley Hall conductivity present in the BN band structure and to the topologically protected kink states that appear in continuum Dirac models with position-dependent masses.

http://pubs.acs.org/doi/abs/10.1021/nl300610w

Ápr. 12. - Ápr. 26 (2012)

Válogatta: Makk Péter

Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3

Dohun Kim, Sungjae Cho, Nicholas P. Butch, Paul Syers, Kevin Kirshenbaum, Shaffique Adam, Johnpierre Paglione & Michael S. Fuhrer

The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states. Although the STI surface state has been studied spectroscopically, for example, by photoemission and scanned probes, transport experiments have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (~ 10 nm), low-doped Bi2Se3 (≈1017 cm−3) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region. A theory of charge disorder in a Dirac band19, 20, 21 explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e2/h per surface) and the residual (puddle) carrier density (0.4×1012 to 4×1012 cm−2). From the measured carrier mobilities 320–1,500 cm2 V−1 s−1, the charged impurity densities 0.5×1013 to 2.3×1013 cm−2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1×1013 to 3×1013 cm−2), identifying dopants as the charged impurities.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2286.html

Observation of the fractional ac Josephson effect: the signature of Majorana particles

Leonid P. Rokhinson, Xinyu Liu, and Jacek K. Furdyna

We report the observation of the fractional ac Josephson effect in hybrid semiconductor/superconductor InSb/Nb nanowire junctions, a hallmark of topological matter. When the junction is irradiated with rf frequency f0 at zero external magnetic field, quantized voltage steps (Shapiro steps) with a height �V = hf0=2e are observed, as is expected for conventional superconductor junctions where the super current is carried by charge-2e Cooper pairs. At high fields the height of the first Shapiro step is doubled to hf0=e. The supercurrent carried by charge-e quasiparticles is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago.

http://arxiv.org/abs/1204.4212

Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device

M. T. Deng, C. L. Yu, G. Y. Huang, M. Larsson, P. Caroff, and H. Q. Xu

... Here, we report on the observation of excitation of Majorana fermions in a Nb-InSb nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed between the two Nb contacts by weak Schottky barriers and is thus in the regime of strong couplings to the contacts. Due to the proximity effect, the InSb nanowire segments covered by superconductor Nb contacts turn to superconductors with a superconducting energy gap. Under an applied magnetic field larger than a critical value for which the Zeeman energy in the InSb nanowire is Ez � ��, the entire InSb nanowire is found to be in a nontrivial topological superconductor phase, supporting a pair of Majorana fermions, and Cooper pairs can transport between the superconductor Nb contacts via the Majorana fermion states. This transport process will be suppressed when the applied magnetic field becomes larger than a second critical value at which the transition to a trivial topological superconductor phase occurs in the system. This physical scenario has been observed in our experiment. We have found that the measured zero-bias conductance for our hybrid device shows a conductance plateau in a range of the applied magnetic field in quasi-particle Coulomb blockade regions. This work provides a simple, solid way of detecting Majorana fermions in solid state systems and should greatly stimulate Majorana fermion research and applications.

http://arxiv.org/abs/1204.4130v1

Coherent quantum phase slip

O. V. Astafiev, L. B. Ioffe, S. Kafanov, Yu. A. Pashkin, K. Yu. Arutyunov, D. Shahar, O. Cohen & J. S. Tsai

A hundred years after the discovery of superconductivity, one fundamental prediction of the theory, coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual1 to the Josephson effect; whereas the latter is a coherent transfer of charges between superconducting leads, the former is a coherent transfer of vortices or fluxes across a superconducting wire. In contrast to previously reported observations of incoherent phase slip, CQPS has been only a subject of theoretical study. Its experimental demonstration is made difficult by quasiparticle dissipation due to gapless excitations in nanowires or in vortex cores. This difficulty might be overcome by using certain strongly disordered superconductors near the superconductor–insulator transition. Here we report direct observation of CQPS in a narrow segment of a superconducting loop made of strongly disordered indium oxide; the effect is made manifest through the superposition of quantum states with different numbers of flux quanta. As with the Josephson effect, our observation should lead to new applications in superconducting electronics and quantum metrology.

http://www.nature.com/nature/journal/v484/n7394/full/nature10930.html

Spintronics reviews in Nat. Materials:

Current-induced torques in magnetic materials (Arne Brataas, Andrew D. Kent and Hideo Ohno) http://www.nature.com/nmat/journal/v11/n5/full/nmat3311.html

Spin Hall effect devices (Tomas Jungwirth, Jörg Wunderlich and Kamil Olejník) http://www.nature.com/nmat/journal/v11/n5/full/nmat3279.html

Spintronics and pseudospintronics in graphene and topological insulators (Dmytro Pesin and Allan H. MacDonald) http://www.nature.com/nmat/journal/v11/n5/full/nmat3305.html

New moves of the spintronics tango (Jairo Sinova and Igor Žutić) http://www.nature.com/nmat/insight/spintronics/index.html

Quantum Hall Effect in Graphene with Superconducting Electrodes

Peter Rickhaus, Markus Weiss,* Laurent Marot, and Christian Schönenberger

We have realized an integer quantum Hall system with superconducting contacts by connecting graphene to niobium electrodes. Below their upper critical field of 4 T, an integer quantum Hall effect coexists with superconductivity in the leads but with a plateau conductance that is larger than in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the graphene−superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor and is less pronounced on the first plateau due to the special nature of the zero energy Landau level in monolayer graphene.

http://dx.doi.org/10.1021/nl204415s

Prospects for Spin-Based Quantum Computing

Christoph Kloeffel and Daniel Loss

Experimental and theoretical progress toward quantum computation with spins in quantum dots (QDs) is reviewed, with particular focus on QDs formed in GaAs heterostructures, on nanowire-based QDs, and on self-assembled QDs. We report on a remarkable evolution of the field where decoherence – one of the main challenges for realizing quantum computers – no longer seems to be the stumbling block it had originally been considered. General concepts, relevant quantities, and basic requirements for spin-based quantum computing are explained; opportunities and challenges of spin-orbit interaction and nuclear spins are reviewed. We discuss recent achievements, present current theoretical proposals, and make several suggestions for further experiments.

http://arxiv.org/abs/1204.5917v1

Electronic properties of graphene: a perspective from scanning tunneling microscopy and magneto-transport. Eva Y. Andrei, Guohong Li and Xu Du

This review covers recent experimental progress in probing the electronic properties of graphene and how they are influenced by various substrates, by the presence of a magnetic field and by the proximity to a superconductor. The focus is on results obtained using scanning tunneling microscopy, spectroscopy, transport and magneto-transport techniques.

http://arxiv.org/abs/1204.4532

Spectroscopy of Spin-Orbit Quantum Bits in Indium Antimonide Nanowires S. Nadj-Perge, V. S. Pribiag, J.W. G. van den Berg, K. Zuo, S. R. Plissard, E. P. A. M. Bakkers, S. M. Frolov, and L. P. Kouwenhoven

A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlettriplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlettriplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.

http://prl.aps.org/abstract/PRL/v108/i16/e166801

First-Order 0-Pi Quantum Phase Transition in the Kondo Regime of a Superconducting Carbon-Nanotube Quantum Dot

Romain Maurand,1 Tobias Meng,2 Edgar Bonet,1 Serge Florens,1 Lae¨titia Marty,1,* and Wolfgang Wernsdorfer1

We study a carbon-nanotube quantum dot embedded in a superconducting-quantum-interference-device loop in order to investigate the competition of strong electron correlations with a proximity effect. Depending on whether local pairing or local magnetism prevails, a superconducting quantum dot will exhibit a positive or a negative supercurrent, referred to as a 0 or � Josephson junction, respectively. In the regime of a strong Coulomb blockade, the 0-to-� transition is typically controlled by a change in the discrete charge state of the dot, from even to odd. In contrast, at a larger tunneling amplitude, the Kondo effect develops for an odd-charge (magnetic) dot in the normal state, and quenches magnetism. In this situation, we find that a first-order 0-to-� quantum phase transition can be triggered at a fixed valence when superconductivity is brought in, due to the competition of the superconducting gap and the Kondo temperature. The superconducting-quantum-interference-device geometry together with the tunability of our device allows the exploration of the associated phase diagram predicted by recent theories. We also report on the observation of anharmonic behavior of the current-phase relation in the transition regime, which we associate with the two accessible superconducting states. Our results finally demonstrate that the spin-singlet nature of the Kondo state helps to enhance the stability of the 0 phase far from the mixed-valence regime in odd-charge superconducting quantum dots.

http://prx.aps.org/abstract/PRX/v2/i1/e011009

Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits

M. D. Shulman, O. E. Dial, S. P. Harvey, H. Bluhm, V. Umansky, A. Yacoby

Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. However, their weak interactions with the environment, which lead to their long coherence times, make interqubit operations challenging. We performed a controlled two-qubit operation between singlet-triplet qubits using a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography, we measured the full density matrix of the system and determined the concurrence and the fidelity of the generated state, providing proof of entanglement.

http://www.sciencemag.org/content/336/6078/202.full

Revealing the Angular Symmetry of Chemical Bonds by Atomic Force Microscopy

Joachim Welker, Franz J. Giessibl

We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.

http://www.sciencemag.org/content/336/6080/444.full

Electrically controlled quantum dot based spin current injector

Szabolcs Csonka , I Weymann and Gergely Zarand

We present a proposal for a fully electrically controllable quantum dot based spin current injector. The device consists of a quantum dot that is strongly coupled to a ferromagnetic electrode on one side and weakly coupled to a nonmagnetic electrode on the other side. The presence of ferromagnetic electrode results in an exchange field that splits the dot level. We show that this exchange-induced splitting can lead to almost full spin polarization of the current owing through the device. Moreover, we also demonstrate that the sign of the polarization can be changed by the gate or the bias voltage within a switching time in the nanosecond range. Thus the proposed device can operate as an electrically controlled, fast switchable spin current source, which can be realized in various state-of-the-art nanostructures.

http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr30399j

Ápr. 6. - Ápr. 12 (2012)

Válogatta: Sárkány Lőrinc

Compensation of Coulomb Blocking and Energy Transfer in the Current Voltage Characteristic of Molecular Conduction Junctions

Guangqi Li, Manmohan S. Shishodia, Boris D. Fainberg, Boris Apter, Michal Oren, Abraham Nitzan, and Mark A. Ratner

We have studied the influence of both exciton effects and Coulomb repulsion on current in molecular nanojunctions. We show that dipolar energy-transfer interactions between the sites in the wire can at high voltage compensate Coulomb blocking for particular relationships between their values. Tuning this relationship may be achieved by using the effect of plasmonic nanostructure on dipolar energy-transfer interactions.

http://pubs.acs.org/doi/abs/10.1021/nl204130d

Ultrafast Relaxation Dynamics via Acoustic Phonons in Carbon Nanotubes

Olga A. Dyatlova, Christopher Köhler, Ermin Malic, Jordi Gomis-Bresco, Janina Maultzsch, Andrey Tsagan-Mandzhiev, Tobias Watermann, Andreas Knorr, and Ulrike Woggon

Carbon nanotubes as one-dimensional nanostructures are ideal model systems to study relaxation channels of excited charged carriers. The understanding of the ultrafast scattering processes is the key for exploiting the huge application potential that nanotubes offer, e.g., for light-emitting and detecting nanoscale electronic devices. In a joint study of two-color pump–probe experiments and microscopic calculations based on the density matrix formalism, we extract, both experimentally and theoretically, a picosecond carrier relaxation dynamics, and ascribe it to the intraband scattering of excited carriers with acoustic phonons. The calculated picosecond relaxation times show a decrease for smaller tube diameters. The best agreement between experiment and theory is obtained for the (8,7) nanotubes with the largest investigated diameter and chiral angle for which the applied zone-folded tight-binding wave functions are a good approximation.

http://pubs.acs.org/doi/abs/10.1021/nl2043997

Quantum Hall effect in exfoliated graphene affected by charged impurities: Metrological measurements

J. Guignard, D. Leprat, D. C. Glattli, F. Schopfer, and W. Poirier

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-μm-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on Si/SiO2 substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature (T<1.3 K), the Hall resistance is measured by comparison with a GaAs-based quantum resistance standard using a cryogenic current comparator. In the low-dissipation limit, it is found quantized within 5 parts in 107 (one standard deviation, 1σ) at the expected rational fractions of the von Klitzing constant, respectively, RK/2 and RK/4 in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the 10−9 accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than 1 μA. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasielastic inter-Landau-level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau-level transition rate and cause small breakdown currents.

http://prb.aps.org/abstract/PRB/v85/i16/e165420

Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon

Patrick Vogt, Paola De Padova, Claudio Quaresima, Jose Avila, Emmanouil Frantzeskakis, Maria Carmen Asensio, Andrea Resta, Bénédicte Ealet, and Guy Le Lay

Because of its unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy in conjunction with calculations based on density functional theory.

http://prl.aps.org/abstract/PRL/v108/i15/e155501

Direct observation of nuclear field fluctuations in single quantum dots

R. Kaji, S. Adachi, H. Sasakura, and S. Muto

The spin interaction between an electron and nuclei was investigated optically in a single self-assembled InAlAs quantum dot (QD). In spin dynamics at the initial stage, the fluctuation of nuclear field and the resulting electron spin relaxation time play a crucial role. We examined a positively charged exciton in a QD to evaluate the key physical quantities directly via the temporal evolution measurements of the Overhauser shift and the degree of circular polarization. In addition, the validity of our used spin dynamics model was discussed in the context of the experimentally obtained key parameters.

http://prb.aps.org/abstract/PRB/v85/i15/e155315

Lagrange formalism of memory circuit elements: Classical and quantum formulations

Guy Z. Cohen, Yuriy V. Pershin, and Massimiliano Di Ventra

The general Lagrange-Euler formalism for the three memory circuit elements, namely, memristive, memcapacitive, and meminductive systems, is introduced. In addition, mutual meminductance, i.e., mutual inductance with a state depending on the past evolution of the system, is defined. The Lagrange-Euler formalism for a general circuit network, the related work-energy theorem, and the generalized Joule's first law are also obtained. Examples of this formalism applied to specific circuits are provided, and the corresponding Hamiltonian and its quantization for the case of nondissipative elements are discussed. The notion of memory quanta, the quantum excitations of the memory degrees of freedom, is presented. Specific examples are used to show that the coupling between these quanta and the well-known charge quanta can lead to a splitting of degenerate levels and to other experimentally observable quantum effects.

http://prb.aps.org/abstract/PRB/v85/i16/e165428

Optical detection of single non-absorbing molecules using the surface plasmon resonance of a gold nanorod

Peter Zijlstra, Pedro M. R. Paulo and Michel Orrit

Existing methods for the optical detection of single molecules require the molecules to absorb light to produce fluorescence or direct absorption signals. This limits the range of species that can be detected, because most molecules are purely refractive. Metal nanoparticles or dielectric resonators can be used to detect non-absorbing molecules because local changes in the refractive index produce a resonance shift. However, current approaches only detect single molecules when the resonance shift is amplified by a highly polarizable label, or by a localized precipitation reaction on the surface of a nanoparticle. Without such amplification, single-molecule events can only be identified in a statistical way. Here, we report the plasmonic detection of single molecules in real time without the need for labelling or amplification. Our sensor consists of a single gold nanorod coated with biotin receptors, and the binding of single proteins is detected by monitoring the plasmon resonance of the nanorod with a sensitive photothermal assay. The sensitivity of our device is ~700 times higher than state-of-the-art plasmon sensors and is intrinsically limited by spectral diffusion of the surface plasmon resonance.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.51.html

Scalable Fabrication of Self-Aligned Graphene Transistors and Circuits on Glass

Lei Liao, Jingwei Bai, Rui Cheng, Hailong Zhou, Lixin Liu, Yuan Liu, Yu Huang, and Xiangfeng Duan

Graphene transistors are of considerable interest for radio frequency (rf) applications. High-frequency graphene transistors with the intrinsic cutoff frequency up to 300 GHz have been demonstrated. However, the graphene transistors reported to date only exhibit a limited extrinsic cutoff frequency up to about 10 GHz, and functional graphene circuits demonstrated so far can merely operate in the tens of megahertz regime, far from the potential the graphene transistors could offer. Here we report a scalable approach to fabricate self-aligned graphene transistors with the extrinsic cutoff frequency exceeding 50 GHz and graphene circuits that can operate in the 1–10 GHz regime. The devices are fabricated on a glass substrate through a self-aligned process by using chemical vapor deposition (CVD) grown graphene and a dielectrophoretic assembled nanowire gate array. The self-aligned process allows the achievement of unprecedented performance in CVD graphene transistors with a highest transconductance of 0.36 mS/μm. The use of an insulating substrate minimizes the parasitic capacitance and has therefore enabled graphene transistors with a record-high extrinsic cutoff frequency (> 50 GHz) achieved to date. The excellent extrinsic cutoff frequency readily allows configuring the graphene transistors into frequency doubling or mixing circuits functioning in the 1–10 GHz regime, a significant advancement over previous reports (20 MHz). The studies open a pathway to scalable fabrication of high-speed graphene transistors and functional circuits and represent a significant step forward to graphene based radio frequency devices.

http://pubs.acs.org/doi/abs/10.1021/nl201922c

High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells

Jong Min Yuk, Jungwon Park, Peter Ercius, Kwanpyo Kim, Daniel J. Hellebusch, Michael F. Crommie, Jeong Yong Lee, A. Zettl, A. Paul Alivisatos

We introduce a new type of liquid cell for in situ transmission electron microscopy (TEM) based on entrapment of a liquid film between layers of graphene. The graphene liquid cell facilitates atomic-level resolution imaging while sustaining the most realistic liquid conditions achievable under electron-beam radiation. We employ this cell to explore the mechanism of colloidal platinum nanocrystal growth. Direct atomic-resolution imaging allows us to visualize critical steps in the process, including site-selective coalescence, structural reshaping after coalescence, and surface faceting.

http://www.sciencemag.org/content/336/6077/61.full

Fundamental temperature-dependent properties of the Si nanocrystal band gap

A. M. Hartel, S. Gutsch, D. Hiller, and M. Zacharias

Ever since the first reports about low-temperature photoluminescence (PL) measurements of Si nanocrystals Si (NCs), a severe deviation from the equations commonly used to describe the temperature dependence of the band-gap energy for bulk materials was reported. Our analysis reveals that the formerly observed deviation is solely attributed to too high excitation power densities that cause a preferential emission enhancement of the smaller NCs within a size distribution, due to the temperature dependence of the radiative exciton lifetime. We report on the successful fit of the temperature-dependent band-gap energy of quantum-confined silicon. By means of four size-controlled Si NC samples (1.5 to 4.5 nm), we are able to prove the validity of these equations down to liquid He temperatures, if sufficiently low excitation power densities (<500 μW/cm2) are used. Thereby, it is shown that the characteristics of the band-gap widening of quantum-confined nanocrystalline Si obeys to the same physical law as the bulk Si crystal. In addition, the previously observed decrease of the PL intensity for decreasing temperatures is demonstrated to have its origin in the same measurement artifact caused by excitation saturation.

http://prb.aps.org/abstract/PRB/v85/i16/e165306

Injection and detection of spin in a semiconductor by tunneling via interface states

R. Jansen, A. M. Deac, H. Saito, and S. Yuasa

Injection and detection of spin accumulation in a semiconductor having localized states at the interface is evaluated. Spin transport from a ferromagnetic contact by sequential, two-step tunneling via interface states is treated not in itself, but in parallel with direct tunneling. The spin accumulation Δμch induced in the semiconductor channel is not suppressed, as previously argued, but genuinely enhanced by the additional spin current via interface states. Spin detection with a ferromagnetic contact yields a weighted average of Δμch and the spin accumulation Δμls in the localized states. In the regime where Δμls/Δμch is largest, the detected spin signal is insensitive to Δμls and the ferromagnet probes the spin accumulation in the semiconductor channel.

http://prb.aps.org/abstract/PRB/v85/i13/e134420

Márc. 30. - Ápr. 5. (2012)

Válogatta: Scherübl Zoltán

Visualizing Electrical Breakdown and ON/OFF States in Electrically Switchable Suspended Graphene Break Junctions

Hang Zhang†, Wenzhong Bao†, Zeng Zhao†, Jhao-Wun Huang†, Brian Standley‡, Gang Liu†, Fenglin Wang†, Philip Kratz†, Lei Jing†, Marc Bockrath*†‡, and Chun Ning Lau*†

Narrow gaps are formed in suspended single- to few-layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with voltages of 2.5 to 4.5 V, corresponding to an ON pulse, and 8 V, corresponding to an OFF pulse. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement and underscores the potential of all-carbon devices for integration with graphene electronics.

http://pubs.acs.org/doi/abs/10.1021/nl203160x

Detection of Vibration-Mode Scattering in Electronic Shot Noise

Manohar Kumar1, Rémi Avriller2,3,4, Alfredo Levy Yeyati2, and Jan M. van Ruitenbeek1

We present shot noise measurements on Au nanowires showing very pronounced vibration-mode features. In accordance to recent theoretical predictions the sign of the inelastic signal, i.e., the signal due to vibration excitations, depends on the transmission probability becoming negative below a certain transmission value. We argue that the negative contribution to noise arises from coherent two-electron processes mediated by electron-phonon scattering and the Pauli exclusion principle. These signals can provide unique information on the local phonon population and lattice temperature of the nanoscale system.

http://prl.aps.org/abstract/PRL/v108/i14/e146602

Pressure Tuning of the Optical Properties of GaAs Nanowires

Ilaria Zardo†‡*, Sara Yazji†, Carlo Marini§, Emanuele Uccelli†, Anna Fontcuberta i Morral, Gerhard Abstreiter†‡, and Paolo Postorino§

The tuning of the optical and electronic properties of semiconductor nanowires can be achieved by crystal phase engineering. Zinc-blende and diamond semiconductors exhibit pressure-induced structural transitions as well as a strong pressure dependence of the band gaps. When reduced to nanoscale dimensions, new phenomena may appear. We demonstrate the tuning of the optical properties of GaAs nanowires and the induction of a phase transition by applying an external pressure. The dependence of the E0 gap on the applied pressure was measured, and a direct-to-indirect transition was found. Resonant Raman scattering was obtained by pressure tuning of the E0 and the E0 + ΔSO gaps with respect to the excitation energy. The resonances of the longitudinal optical modes LO and 2LO indicate the presence of electron–phonon Fröhlich interactions. These measurements show for the first time a variation of ionicity in GaAs when in nanowire form. Furthermore, the dependence of the lattice constant on applied pressure was estimated. Finally, we found a clear indication of a structural transition above 16 GPa.

http://pubs.acs.org/doi/abs/10.1021/nn300228u

Transport gap in suspended bilayer graphene at zero magnetic field

A. Veligura1,*, H. J. van Elferen2, N. Tombros1, J. C. Maan2, U. Zeitler2, and B. J. van Wees1

We report a change of three orders of magnitude in the resistance of a suspended bilayer graphene flake which varies from a few kΩ in the high-carrier-density regime to several MΩ around the charge neutrality point (CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of quantum Hall plateaus appearing at filling factor ν=2 followed by ν=1 suggests that the observed gap is caused by the symmetry breaking of the lowest Landau level. Investigation of the gap in a tilted magnetic fields indicates that the resistance at the CNP shows a weak linear decrease for increasing total magnetic field. Those observations are in agreement with a spontaneous valley splitting at zero magnetic field followed by splitting of the spins originating from different valleys with increasing magnetic field. Both the transport gap and B field response point toward the spin-polarized layer-antiferromagnetic state as the ground state in the bilayer graphene sample. The observed nontrivial dependence of the gap value on the normal component of B suggests possible exchange mechanisms in the system.

http://prb.aps.org/abstract/PRB/v85/i15/e155412

Intertwining of Zeeman and Coulomb interactions on excitons in highly symmetric semiconductor quantum dots

D. Y. Oberli

We present an experimental study and develop a group theoretical analysis of the Zeeman effect on excitons in pyramidal semiconductor quantum dots possessing the symmetries of the C3v point group. The magnetic field dependence of the emission pattern originating from neutral exciton states is investigated in both the Faraday and Voigt configurations. The Zeeman doublet splitting of the “bright” exciton states varies linearly with the magnetic field strength in each configuration while the intensity of the “dark” exciton transitions exhibit a nonlinear dependence. We demonstrate that these observations originate from the intertwining of the Zeeman and Coulomb interactions, which provides clear spectral signatures of this effect for highly symmetric quantum dots. We uncover a large anisotropy of the Zeeman doublet splittings for longitudinal and transverse magnetic fields, revealing the ubiquitous role of a symmetry elevation in our pyramidal quantum dots. These results suggest that the common description of the Zeeman effect based on effective g factors for electrons and holes must be revised when dealing with exciton complexes.

http://prb.aps.org/abstract/PRB/v85/i15/e155305

Fast Hybrid Silicon Double-Quantum-Dot Qubit

Zhan Shi1, C. B. Simmons1, J. R. Prance1, John King Gamble1, Teck Seng Koh1, Yun-Pil Shim1, Xuedong Hu2, D. E. Savage1, M. G. Lagally1, M. A. Eriksson1, Mark Friesen1, and S. N. Coppersmith1

We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S2=3/4 (S=1/2) and Sz=-1/2, with the two different states being singlet and triplet in the doubly occupied dot. Gate operations can be implemented electrically and the qubit is highly tunable, enabling fast implementation of one- and two-qubit gates in a simpler geometry and with fewer operations than in other proposed quantum dot qubit architectures with fast operations. Moreover, the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.

http://prl.aps.org/abstract/PRL/v108/i14/e140503

Spin Polarization of the 12/5 Fractional Quantum Hall Effect

Chi Zhang, Chao Huan, J. S. Xia, N. S. Sullivan, W. Pan, K.W. Baldwin, K. W. West, L. N. Pfeiffer, D. C. Tsuiű

We have carried out tilt magnetic field (B) studies of the \nu=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (\theta). It first increases sharply with increasing \theta, reaches a maximal value of ~ 70 ohms at \theta ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on \theta, the 12/5 activation energy (\Delta_{12/5}) also shows a non-monotonic tilt dependence. \Delta_{12/5} first decreases with increasing \theta. Around \theta = 14^{o}, \Delta_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, \Delta_{12/5} reemerges and increases with \theta. This tilt B dependence at \nu=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.

http://arxiv.org/abs/1204.0560

Transport Properties of Graphene Nanoribbon Transistors on Transport Properties of Graphene Nanoribbon Transistors on Chemical-Vapor-Deposition Grown Wafer-Scale Graphene

Wan Sik Hwang, Kristof Tahy, Xuesong Li, Huili (Grace) Xing, Alan C. Seabaugh, Chun-Yung Sung, Debdeep Jena

Graphene nanoribbon (GNR) field-effect transistors (FETs) with widths down to 12 nm have been fabricated by electron beam lithography using a wafer-scale chemical vapor deposition (CVD) process to form the graphene. The GNR FETs show drain-current modulation of approximately 10 at 300 K, increasing to nearly 106 at 4 K. The strong temperature dependence of the minimum current indicates the opening of a bandgap for CVD-grown GNR-FETs. The extracted bandgap is estimated to be around 0.1 eV by differential conductance methods. This work highlights the development of CVD-grown large-area graphene and demonstrates the opening of a bandgap in nanoribbon transistors.

http://arxiv.org/abs/1204.0499

Epitaxial Growth of a Silicene Sheet

Boubekeur Lalmi, Hamid Oughaddoub, Hanna Enriquezb, Abdelkader Karae, Sébastien Vizzini, Bénidicte Ealet, Bernard Aufray

Using atomic resolved scanning tunneling microscopy, we present here the experimental evidence of a silicene sheet (graphene like structure) epitaxially grown on a close-packed silver surface (Ag(111)). This has been achieved via direct condensation of a silicon atomic flux onto the single-crystal substrate in ultra-high vacuum conditions. A highly ordered silicon structure, arranged within a honeycomb lattice is synthesized and presenting two silicon sub-lattices occupying positions at different heights (0.02 nm) indicating possible sp2-sp3 hybridizations.

http://arxiv.org/abs/1204.0523

Electronic and transport properties of azobenzene monolayer junctions as molecular switches

Yan Wang, Hai-Ping Cheng

We investigate from first-principles the change in transport properties of a two-dimensional azobenzene monolayer sandwiched between two Au electrodes that undergoes molecular switching. We focus on transport differences between a chemisorbed and physisorbed top monolayer-electrode contact. The conductance of the monolayer junction with a chemisorbed top contact is higher in \textit{trans} configuration, in agreement with the previous theoretical predictions of one-dimensional single molecule junctions. However, with a physisorbed top contact, the "ON" state with larger conductance is associated with the \textit{cis} configuration due to a reduced effective tunneling pathway by switching from \textit{trans} to \textit{cis}, which successfully explains recently experimental measurements of azobenzene monolayer junctions. A simple model is developed to explain electron transmission across subsystems in the molecular junction. We also discuss the effects of monolayer packing density, molecule tilt angle, and contact geometry on the calculated transmission functions. In particular, we find that a tip-like contact with chemisorption significantly affects the electric current through the \textit{cis} monolayer, leading to highly asymmetric current-voltage characteristics as well as large negative differential resistance behavior.

http://arxiv.org/abs/1203.6862

Quantized Charge Pumping through a Carbon Nanotube Double Quantum Dot

S. J. Chorley, J. Frake, C. G. Smith, G. A. C. Jones, M. R. Buitelaar

We demonstrate single-electron pumping in a gate-defined carbon nanotube double quantum dot. By periodic modulation of the potentials of the two quantum dots we move the system around charge triple points and transport exactly one electron or hole per cycle. We investigate the pumping as a function of the modulation frequency and amplitude and observe good current quantization up to frequencies of 18 MHz where rectification effects cause the mechanism to break down.

http://arxiv.org/abs/1204.1044

Temperature dependence of dynamic nuclear polarization and its effect on electron spin relaxation and dephasing in InAs/GaAs quantum dots

J. Beyer1, Y. Puttisong1, I. A. Buyanova1, S. Suraprapapich2, C. W. Tu2, and W. M. Chen1

Electron spin dephasing and relaxation due to hyperfine interaction with nuclear spins is studied in an InAs/GaAs quantum dot ensemble as a function of temperature up to 85 K, in an applied longitudinal magnetic field. The extent of hyperfine-induced dephasing is found to decrease, whereas dynamic nuclear polarization increases with increasing temperature. We attribute both effects to an accelerating electron spin relaxation through phonon-assisted electron-nuclear spin flip-flops driven by hyperfine interactions, which could become the dominating contribution to electron spin depolarization at high temperatures.

http://apl.aip.org/resource/1/applab/v100/i14/p143105_s1?isAuthorized=no

Readout of carbon nanotube vibrations based on spin-phonon coupling

C. Ohm1,2, C. Stampfer2,3,4, J. Splettstoesser1,2, and M. R. Wegewijs1,2,4

We propose a scheme for spin-based detection of the bending motion in suspended carbon-nanotubes, using the curvature-induced spin-orbit interaction. We show that the resulting effective spin-phonon coupling can be used to down-convert the high-frequency vibration-modulated spin-orbit field to spin-flip processes at a much lower frequency. This vibration-induced spin-resonance can be controlled with an axial magnetic field. We propose a Pauli spin blockade readout scheme and predict that the leakage current shows pronounced peaks as a function of the external magnetic field. Whereas the resonant peaks allow for frequency readout, the slightly off-resonant current is sensitive to the vibration amplitude.

http://apl.aip.org/resource/1/applab/v100/i14/p143103_s1?isAuthorized=no

Molecular motors: Myosin shifts into reverse gear

Wilhelm J. Walter & Stefan Diez

A motor protein can be made to walk in either direction along a filamentous track by adjusting the concentration of calcium ions in the surrounding solution.

http://www.nature.com/nnano/journal/v7/n4/full/nnano.2012.49.html

Tunable Spin–Orbit Interaction in Trilayer Graphene Exemplified in Electric-Double-Layer Transistors

Zhuoyu Chen†, Hongtao Yuan*†, Yanfeng Zhang‡, Kentaro Nomura§, Teng Gao‡, Yabo Gao‡, Hidekazu Shimotani†, Zhongfan Liu‡, and Yoshihiro Iwasa*†§

Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin–orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L and spin relaxation length Lso of carriers can be effectively modulated with gate bias. As a direct result, SOI-induced weak antilocalization (WAL), together with a crossover from WAL to weak localization (WL), was observed at near-zero magnetic field. Interestingly, among existing localization models, only the Iordanskii–Lyanda-Geller–Pikus theory can successfully reproduce the obtained magnetoconductance well, serving as evidence for gate tuning of the weak but distinct SOI in graphene. Realization of SOI and its large tunability in the trilayer graphene EDLTs provides us with a possibility to electrically manipulate spin precession in graphene systems without ferromagnetics.

http://pubs.acs.org/doi/abs/10.1021/nl204012c

Raman sensitivity to crystal structure in InAs nanowires

Jaya Kumar Panda1, Anushree Roy1, Achintya Singha2, Mauro Gemmi3, Daniele Ercolani4, Vittorio Pellegrini4, and Lucia Sorba4

We report electron transmission and Raman spectroscopy study of InAs nanowires. We demonstrate that the temperature dependent behavior of optical phonon energies can be used to determine the relative wurtzite fraction in the InAs nanowires. Furthermore, we propose that the interfacial strain between zincblende and wurtzite phases along the length of the wires manifests in the temperature-evolution of the phonon linewidths. From these studies, temperature-dependent Raman measurements emerge as a non-invasive method to study polytypism in such nanowires.

http://apl.aip.org/resource/1/applab/v100/i14/p143101_s1?isAuthorized=no

Theory of ferromagnetic unconventional superconductors with spin-triplet electron pairing

Dimo I. Uzunov

A general phenomenological theory is presented for the phase behavior of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory describes in details the temperature-pressure phase diagrams of real inter-metallic compounds exhibiting the remarkable phenomenon of coexistence of spontaneous magnetic moment of the itinerant electrons and spin-triplet superconductivity. The quantum phase transitions which may occur in these systems are also described. The theory allows for a classification of these itinerant ferromagnetic superconductors in two types: type I and type II. The classification is based on quantitative criteria.The comparison of theory and experiment is performed and outstanding problems are discussed.

http://arxiv.org/abs/1204.1007

Márc. 1. - Márc. 22. (2012)

Válogatta: Magyarkuti András

Dynamical Coulomb Blockade Observed in Nanosized Electrical Contacts

Christophe Brun1,2, Konrad H. Müller3, I-Po Hong1,4, François Patthey1, Christian Flindt3, and Wolf-Dieter Schneider1

Electrical contacts between nanoengineered systems are expected to constitute the basic building blocks of future nanoscale electronics. However, the accurate characterization and understanding of electrical contacts at the nanoscale is an experimentally challenging task. Here, we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nanocontacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island-substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements.

http://prl.aps.org/abstract/PRL/v108/i12/e126802

Relativistic Hall Effect

Konstantin Y. Bliokh1,2 and Franco Nori1,3

We consider the relativistic deformation of quantum waves and mechanical bodies carrying intrinsic angular momentum (AM). When observed in a moving reference frame, the centroid of the object undergoes an AM-dependent transverse shift. This is the relativistic analogue of the spin-Hall effect, which occurs in free space without any external fields. Remarkably, the shifts of the geometric and energy centroids differ by a factor of 2, and both centroids are crucial for the Lorentz transformations of the AM tensor. We examine manifestations of the relativistic Hall effect in quantum vortices and mechanical flywheels and also discuss various fundamental aspects of this phenomenon. The perfect agreement of quantum and relativistic approaches allows applications at strikingly different scales, from elementary spinning particles, through classical light, to rotating black holes.

http://prl.aps.org/abstract/PRL/v108/i12/e120403

Enhanced Carrier Transport along Edges of Graphene Devices

Jungseok Chae†§, Suyong Jung‡§, Sungjong Woo, Hongwoo Baek†, Jeonghoon Ha†, Young Jae Song‡§, Young-Woo Son, Nikolai B. Zhitenev‡, Joseph A. Stroscio‡, and Young Kuk†

The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip.

http://pubs.acs.org/doi/abs/10.1021/nl2041222

Highly Efficient Charge Separation and Collection across in Situ Doped Axial VLS-Grown Si Nanowire p–n Junctions

A. D. Mohite*†§, D. E. Perea†‡, S. Singh†§, S. A. Dayeh†‡, I. H. Campbell‡, S. T. Picraux*†‡, and H. Htoon

VLS-grown semiconductor nanowires have emerged as a viable prospect for future solar-based energy applications. In this paper, we report highly efficient charge separation and collection across in situ doped Si p–n junction nanowires with a diameter <100 nm grown in a cold wall CVD reactor. Our photoexcitation measurements indicate an internal quantum efficiency of 50%, whereas scanning photocurrent microscopy measurements reveal effective minority carrier diffusion lengths of 1.0 μm for electrons and 0.66 μm for holes for as-grown Si nanowires (dNW ≈ 65–80 nm), which are an order of magnitude larger than those previously reported for nanowires of similar diameter. Further analysis reveals that the strong suppression of surface recombination is mainly responsible for these relatively long diffusion lengths, with surface recombination velocities (S) calculated to be 2 orders of magnitude lower than found previously for as-grown nanowires, all of which used hot wall reactors. The degree of surface passivation achieved in our as-grown nanowires is comparable to or better than that achieved for nanowires in prior studies at significantly larger diameters. We suggest that the dramatically improved surface recombination velocities may result from the reduced sidewall reactions and deposition in our cold wall CVD reactor.

http://pubs.acs.org/doi/abs/10.1021/nl204505p

Scanning probe microscopy: Seeing the charge within

Peter Grutter

The distribution of electric charge within a single naphthalocyanine molecule has been revealed by researchers using a combination of three types of microscopy and theoretical modelling.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.43.html

Quantum Hall Effect in Graphene with Superconducting Electrodes

Peter Rickhaus, Markus Weiss*, Laurent Marot, and Christian Schönenberger

We have realized an integer quantum Hall system with superconducting contacts by connecting graphene to niobium electrodes. Below their upper critical field of 4 T, an integer quantum Hall effect coexists with superconductivity in the leads but with a plateau conductance that is larger than in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the graphene–superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor and is less pronounced on the first plateau due to the special nature of the zero energy Landau level in monolayer graphene.

http://pubs.acs.org/doi/abs/10.1021/nl204415s

Direct Measurement of the Fermi Energy in Graphene Using a Double-Layer Heterostructure

Seyoung Kim1, Insun Jo2, D. C. Dillen1, D. A. Ferrer1, B. Fallahazad1, Z. Yao2, S. K. Banerjee1, and E. Tutuc1

We describe a technique which allows a direct measurement of the relative Fermi energy in an electron system by employing a double-layer heterostructure. We illustrate this method by using a graphene double layer to probe the Fermi energy as a function of carrier density in monolayer graphene, at zero and in high magnetic fields. This technique allows us to determine the Fermi velocity, Landau level spacing, and Landau level broadening. We find that the N=0 Landau level broadening is larger by comparison to the broadening of upper and lower Landau levels.

http://prl.aps.org/abstract/PRL/v108/i11/e116404

Majorana Fermions and Exotic Surface Andreev Bound States in Topological Superconductors: Application to CuxBi2Se3

Timothy H. Hsieh and Liang Fu

The recently discovered superconductor CuxBi2Se3 is a candidate for three-dimensional time-reversal-invariant topological superconductors, which are predicted to have robust surface Andreev bound states hosting massless Majorana fermions. In this work, we analytically and numerically find the linearly dispersing Majorana fermions at k=0, which smoothly evolve into a new branch of gapless surface Andreev bound states near the Fermi momentum. The latter is a new type of Andreev bound states resulting from both the nontrivial band structure and the odd-parity pairing symmetry. The tunneling spectra of these surface Andreev bound states agree well with a recent point-contact spectroscopy experiment [ S. Sasaki et al. Phys. Rev. Lett. 107 217001 (2011)] and yield additional predictions for low temperature tunneling and photoemission experiments.

http://prl.aps.org/abstract/PRL/v108/i10/e107005

Direct Observation of Inhomogeneous Solid Electrolyte Interphase on MnO Anode with Atomic Force Microscopy and Spectroscopy

Jie Zhang†§, Rui Wang‡, Xiaocheng Yang†, Wei Lu*†, Xiaodong Wu†, Xiaoping Wang§, Hong Li‡, and Liwei Chen

Solid electrolyte interphase (SEI) is an in situ formed thin coating on lithium ion battery (LIB) electrodes. The mechanical property of SEI largely defines the cycling performance and the safety of LIBs but has been rarely investigated. Here, we report quantitatively the Young’s modulus of SEI films on MnO anodes. The inhomogeneity of SEI film in morphology, structure, and mechanical properties provides new insights to the evolution of SEI on electrodes. Furthermore, the quantitative methodology established in this study opens a new approach to direct investigation of SEI properties in various electrode materials systems.

http://pubs.acs.org/doi/abs/10.1021/nl300570d

Febr. 24. - Febr. 31. (2012)

Válogatta: Fülöp Gergő

Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures

L. Britnell, R. V. Gorbachev2, R. Jalil2, B. D. Belle2, F. Schedin2, A. Mishchenko1, T. Georgiou1, M. I. Katsnelson3, L. Eaves4, S. V. Morozov5, N. M. R. Peres6,7, J. Leist8, A. K. Geim1,2,*, K. S. Novoselov1,*, L. A. Ponomarenko1,*

An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of ≈50 and ≈10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration.

http://www.sciencemag.org/content/335/6071/947.abstract

Imaging the charge distribution within a single molecule

Fabian Mohn, Leo Gross, Nikolaj Moll & Gerhard Meyer

[...] Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches13) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.20.html

A Chemically-Responsive Nanojunction within a Silver Nanowire

Wendong Xing, Jun Hu, Sheng-Chin Kung, Keith C. Donavan†, Wenbo Yan†, Ruqian Wu, and Reginald M. Penner

The formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A silver nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition method. A 1–5 nm gap was formed in this wire by electromigration. Finally, this gap was reconnected by applying a voltage ramp to the nanowire resulting in the formation of a resistive, ohmic CRJ. Exposure of this CRJ-containing nanowire to ammonia (NH3) induced a rapid (<30 s) and reversible resistance change that was as large as ΔR/R0 = (+)138% in 7% NH3 and observable down to 500 ppm NH3. Exposure to water vapor produced a weaker resistance increase of ΔR/R0,H2O = (+)10–15% (for 2.3% water) while nitrogen dioxide (NO2) exposure induced a stronger concentration-normalized resistance decrease of ΔR/R0,NO2 = (−)10–15% (for 500 ppm NO2). The proposed mechanism of the resistance response for a CRJ, supported by temperature-dependent measurements of the conductivity for CRJs and density functional theory calculations, is that semiconducting p-type AgxO is formed within the CRJ and the binding of molecules to this AgxO modulates its electrical resistance.

http://pubs.acs.org/doi/full/10.1021/nl300427w

Topographic and Spectroscopic Characterization of Electronic Edge States in CVD Grown Graphene Nanoribbons

Minghu Pan, E. Costa Girão, Xiaoting Jia, Sreekar Bhaviripudi, Qing Li, Jing Kong, V. Meunier, and Mildred S. Dresselhaus

We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in as-grown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with well-defined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron–electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a reconstructed edge structure composed of 5–7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons.

http://pubs.acs.org/doi/full/10.1021/nl204392s

Febr. 16. - Febr. 23. (2012)

Válogatta: Gubicza Ági

A single-atom transistor

Martin Fuechsle, Jill A. Miwa, Suddhasatta Mahapatra, Hoon Ryu, Sunhee Lee, Oliver Warschkow, Lloyd C. L. Hollenberg, Gerhard Klimeck & Michelle Y. Simmons doi:10.1038/nnano.2012.21

Published online 19 February 2012

The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope can manipulate individual atoms and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces, but the fabrication of working devices—such as transistors with extremely short gate lengths, spin-based quantum computers and solitary dopant optoelectronic devices—requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2012.21.html

Thermal Stability and Surface Passivation of Ge Nanowires Coated by Epitaxial SiGe Shells

Shu Hu†, Yoko Kawamura, Kevin C. Y. Huang, Yanying Li, Ann F. Marshall, Kohei M. Itoh, Mark L. Brongersma, and Paul C. McIntyre DOI: 10.1021/nl204053w

Publication Date (Web): February 24, 2012

Epitaxial growth of a highly strained, coherent SiGe alloy shell around a Ge nanowire core is investigated as a method to achieve surface passivation and carrier confinement, important in realizing nanowire devices. The high photoluminescence intensity observed from the core–shell nanowires with spectral features similar to that of bulk Ge indicates effective surface passivation. Thermal stability of these core–shell heterostructures has been systematically investigated, with a method demonstrated to avoid misfit strain relaxation during postgrowth annealing.

http://pubs.acs.org/doi/abs/10.1021/nl204053w

Low-Temperature Chemical Vapor Deposition Growth of Graphene from Toluene on Electropolished Copper Foils

Bin Zhang, Wi Hyoung Lee, Richard Piner, Iskandar Kholmanov, Yaping Wu, Huifeng Li, Hengxing Ji, and Rodney S Ruoff DOI: 10.1021/nn204827h

Publication Date (Web): February 17, 2012

A two-step CVD route with toluene as the carbon precursor was used to grow continuous large-area monolayer graphene films on a very flat, electropolished Cu foil surface at 600 °C, lower than any temperature reported to date for growing continuous monolayer graphene. Graphene coverage is higher on the surface of electropolished Cu foil than that on the unelectropolished one under the same growth conditions. The measured hole and electron mobilities of the monolayer graphene grown at 600 °C were 811 and 190 cm2/(V·s), respectively, and the shift of the Dirac point was 18 V. The asymmetry in carrier mobilities can be attributed to extrinsic doping during the growth or transfer. The optical transmittance of graphene at 550 nm was 97.33%, confirming it was a monolayer, and the sheet resistance was ~8.02 × 103 Ω/□.

http://pubs.acs.org/doi/abs/10.1021/nn204827h

Controlled Synthesis of Compositionally Tunable Ternary PbSexS1–x as Well as Binary PbSe and PbS Nanowires

Anthony C. Onicha, Nattasamon Petchsang, Thomas H. Kosel, and Masaru Kuno DOI: 10.1021/nn300373w

Publication Date (Web): February 16, 2012

High-quality compositionally tunable ternary PbSexS1–x (x = 0.23, 0.39, 0.49, 0.68, and 0.90) nanowires (NWs) and their binary analogues have been grown using solution–liquid–solid growth with lead(II) diethyldithiocarbamate, Pb(S2CNEt2)2, and lead(II) imido(bis(selenodiisopropylphosphinate)), Pb((SePiPr2)2N)2, as single-source precursors. The alloyed nature of PbSexS1–x wires was confirmed using ensemble X-ray diffraction and energy dispersive X-ray spectroscopy (EDXS). Single NW EDXS line scans taken along the length of individual wires show no compositional gradients. NW compositions were independently confirmed using inductively coupled plasma atomic emission spectroscopy. Slight stoichiometric deviations occur but never exceed 13.3% of the expected composition, based on the amount of introduced precursor. In all cases, resulting nanowires have been characterized using transmission electron microscopy. Mean diameters are between 9 and 15 nm with accompanying lengths that range from 4 to 10 μm. Associated selected area electron diffraction patterns indicate that the PbSexS1–x, PbSe, and PbS NWs all possess the same <002> growth direction, with diffraction patterns consistent with an underlying rock salt crystal structure.

http://pubs.acs.org/doi/abs/10.1021/nn300373w

Tunable Band Gaps and p-Type Transport Properties of Boron-Doped Graphenes by Controllable Ion Doping Using Reactive Microwave Plasma

Yong-Bing Tang, Li-Chang Yin, Yang Yang, Xiang-Hui Bo, Yu-Lin Cao, Hong-En Wang, Wen-Jun Zhang, Igor Bello, Shuit-Tong Lee, Hui-Ming Cheng, and Chun-Sing Lee DOI: 10.1021/nn3005262

Publication Date (Web): February 21, 2012

We report tunable band gaps and transport properties of B-doped graphenes that were achieved via controllable doping through reaction with the ion atmosphere of trimethylboron decomposed by microwave plasma. Both electron energy loss spectroscopy and X-ray photoemission spectroscopy analyses of the graphene reacted with ion atmosphere showed that B atoms are substitutionally incorporated into graphenes without segregation of B domains. The B content was adjusted over a range of 0–13.85 atom % by controlling the ion reaction time, from which the doping effects on transport properties were quantitatively evaluated. Electrical measurements from graphene field-effect transistors show that the B-doped graphenes have a distinct p-type conductivity with a current on/off ratio higher than 102. Especially, the band gap of graphenes is tuned from 0 to ~0.54 eV with increasing B content, leading to a series of modulated transport properties. We believe the controllable doping for graphenes with predictable transport properties may pave a way for the development of graphene-based devices.

http://pubs.acs.org/doi/abs/10.1021/nn3005262

Dynamic Tuning and Symmetry Lowering of Fano Resonance in Plasmonic Nanostructure

Yonghao Cui, Jianhong Zhou, Venkata A. Tamma, and Wounjhang Park DOI: 10.1021/nn204647b

Publication Date (Web): February 16, 2012

We present dynamic tuning and symmetry lowering of Fano resonances in gold heptamers accomplished by applying uniaxial mechanical stress. The flexible heptamer structure was obtained by embedding the seven-gold-nanocylinder complex in a polydimethylsiloxane membrane. Under uniaxial stress, the Fano resonance exhibited opposite spectral shifts for the two orthogonal polarizations parallel and perpendicular to the mechanical stress. Furthermore, a new resonance was observed for polarization parallel to the mechanical stress but not for the perpendicular polarization. The experimental results showed good agreement with the numerical simulations. A detailed group theoretical analysis showed that the symmetry lowering caused by the mechanical stress not only splits the originally degenerate mode but also modifies the originally optically inactive mode into an optically active mode, which then interacts strongly with a closely spaced mode and exhibits anticrossing behavior. The symmetry tuning enabled by applying mechanical stress is a simple and efficient way to engineer the nature of coupled plasmon resonances in complex nanostructures. The mechanically tunable plasmonic nanostructures also provide an excellent platform for dynamically tunable nanophotonic devices such as tunable filters and sensors.

http://pubs.acs.org/doi/abs/10.1021/nn204647b

Wetting and Interfacial Properties of Water Nanodroplets in Contact with Graphene and Monolayer Boron–Nitride Sheets

Hui Li and Xiao Cheng Zeng DOI: 10.1021/nn204661d

Publication Date (Web): February 22, 2012

Born–Oppenheim quantum molecular dynamics (QMD) simulations are performed to investigate wetting, diffusive, and interfacial properties of water nanodroplets in contact with a graphene sheet or a monolayer boron–nitride (BN) sheet. Contact angles of the water nanodroplets on the two sheets are computed for the first time using QMD simulations. Structural and dynamic properties of the water droplets near the graphene or BN sheet are also studied to gain insights into the interfacial interaction between the water droplet and the substrate. QMD simulation results are compared with those from previous classic MD simulations and with the experimental measurements. The QMD simulations show that the graphene sheet yields a contact angle of 87°, while the monolayer BN sheet gives rise to a contact angle of 86°. Hence, like graphene, the monolayer BN sheet is also weakly hydrophobic, even though the BN bonds entail a large local dipole moment. QMD simulations also show that the interfacial water can induce net positive charges on the contacting surface of the graphene and monolayer BN sheets, and such charge induction may affect electronic structure of the contacting graphene in view that graphene is a semimetal. Contact angles of nanodroplets of water in a supercooled state on the graphene are also computed. It is found that under the supercooled condition, water nanodroplets exhibit an appreciably larger contact angle than under the ambient condition.

http://pubs.acs.org/doi/abs/10.1021/nn204661d

Second and higher harmonics generation with memristive systems

Guy Z. Cohen, Yuriy V. Pershin, Massimiliano Di Ventra

Submitted on 21 Feb 2012

We show that memristive systems can be used very efficiently to generate passively both double and higher frequency harmonics. A technique for maximizing the power conversion efficiency into any given harmonic is developed and applied to a single memristive system and memristive bridge circuits. We find much higher rates of power conversion compared to the standard diode bridge, with the memristive bridge more efficient for second and higher harmonics generation compared to the single memristive system. The memristive bridge circuit optimized for second harmonic generation behaves as a two-quarter-wave rectifier.

http://arxiv.org/abs/1202.4727

Nuclear spin physics in quantum dots: an optical investigation

Bernhard Urbaszek, Xavier Marie, Thierry Amand, Olivier Krebs, Paul Voisin, Patrick Maletinsky, Alexander Hogele, Atac Imamoglu

Submitted on 21 Feb 2012

The mesoscopic spin system formed by the 104 − 106 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic, coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.

http://arxiv.org/abs/1202.4637

Dynamics of superconducting nanowires shunted with an external resistor

Matthew W. Brenner, Dibyendu Roy, Nayana Shah, Alexey Bezryadin

Submitted on 21 Feb 2012

We present the first study of superconducting nanowires shunted with an external resistor, geared towards understanding and controlling coherence and dissipation in nanowires. The dynamics is probed by measuring the evolution of the V -I characteristics and the distributions of switching and retrapping currents upon varying the shunt resistor and temperature. Theoretical analysis of the experiments indicates that as the value of the shunt resistance is decreased, the dynamics turns more coherent presumably due to stabilization of phase-slip centers in the wire and furthermore the switching current approaches the Bardeen’s prediction for equilibrium depairing current. By a detailed comparison between theory and experimental, we make headway into identifying regimes in which the quasi-one-dimensional wire can effectively be described by a zero-dimensional circuit model analogous to the RCSJ (resistively and capacitively shunted Josephson junction) model of Stewart and McCumber. Besides its fundamental significance, our study has implications for a range of promising technological applications.

http://arxiv.org/abs/1202.4526

Counting statistics in an InAs nanowire quantum dot with a vertically coupled charge detector

Theodore Choi, Thomas Ihn, Silke Schön, Klaus Ensslin

Submitted on 20 Feb 2012

A gate-defined quantum dot in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time- correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kB T is comparable to the single- particle level spacing in the dot.

http://arxiv.org/abs/1202.4273

Stability of Spinmotive Force in Perpendicularly Magnetized Nanowires under High Magnetic Fields

Yuta Yamane, Jun'ichi Ieda, Sadamichi Maekawa

Submitted on 20 Feb 2012

Spinmotive force induced by domain wall motion in perpendicularly magnetized nanowires is numerically demonstrated. We show that by use of nanowires with large magnetic anisotropy, high stability of spinmotive force can be achieved under strong magnetic fields. We observe tens of μV of the spinmotive force in a multilayered Co/Ni nanowire, and even a several hundred μV in a L10 -ordered FePt nanowire, which is two orders of magnitude larger than that reported so far in permalloy nanowires. The narrow structure and low mobility of a domain wall under magnetic fields in perpendicularly magnetized nanowires permits downsizing of spinmotive force devices.

http://arxiv.org/abs/1202.4256

Angle dependent conductance in graphene

C. H. Fuentevilla and J.D. Lejarreta , C. Cobaleda and E. Diez

Submitted on 19 Feb 2012

In this paper, we study a theoretical method to calculate the conductance across a square barrier potential in monolayer graphene. We have obtained an analytical expression for the transmission coefficient across a potential barrier for monolayer graphene. Using the transmission coefficient obtained we have an analytical expression for the conductance. This expression will be used to calculate the conductance in the case in which there is a potential barrier, which in our case will modelise the behaviour of a top gate voltage of a field effect transistor. Once this analysis has been performed we study the scenario in which carriers scatter with the potential barrier with different incidence angles and we have found that for any incident angle an effective gap is induced.

http://arxiv.org/abs/1202.4145

Competition for Graphene: Graphynes with Direction-Dependent Dirac Cones

Daniel Malko, Christian Neiss, Francesc Viñes, and Andreas Görling

Phys. Rev. Lett. 108, 086804 (2012)

The existence of Dirac cones in the band structure of two-dimensional materials accompanied by unprecedented electronic properties is considered to be a unique feature of graphene related to its hexagonal symmetry. Here, we present other two-dimensional carbon materials, graphynes, that also possess Dirac cones according to first-principles electronic structure calculations. One of these materials, 6,6,12-graphyne, does not have hexagonal symmetry and features two self-doped nonequivalent distorted Dirac cones suggesting electronic properties even more amazing than that of graphene.

http://prl.aps.org/abstract/PRL/v108/i8/e086804

Spin-Selective Kondo Insulator: Cooperation of Ferromagnetism and the Kondo Effect

Robert Peters and Norio Kawakami

Phys. Rev. Lett. 108, 086402 (2012)

We propose the notion of a spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic while the minority-spin electrons show insulating behavior. The resulting Kondo gap in the minority-spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically induced commensurability for a combination of minority-spin electrons and parts of localized spins. Furthermore, this mechanism predicts a nontrivial relation between the macroscopic quantities such as electron magnetization, spin polarization, and electron filling.

http://prl.aps.org/abstract/PRL/v108/i8/e086402

Redshift of Excitons in Carbon Nanotubes Caused by the Environment Polarizability

Michael Rohlfing

Phys. Rev. Lett. 108, 087402 (2012)

Optical excitations of molecular systems can be modified by their physical environment. We analyze the underlying mechanisms within many-body perturbation theory, which is particularly suited to study nonlocal polarizability effects on the electronic structure. Here we focus on the example of a semiconducting carbon nanotube, which observes redshifts of its excitons when the tube is touched by another nanotube or other physisorbates. We show that the redshifts mostly result from the polarizability of the attached ad system. Electronic coupling may enhance the redshifts, but depends very sensitively on the structural details of the contact.

http://prl.aps.org/abstract/PRL/v108/i8/e087402

Twisting graphene nanoribbons into carbon nanotubes

O. O. Kit, T. Tallinen, L. Mahadevan, J. Timonen, and P. Koskinen

Phys. Rev. B 85, 085428 (2012)

Although carbon nanotubes consist of honeycomb carbon, they have never been fabricated from graphene directly. Here, it is shown by quantum molecular-dynamics simulations and classical continuum-elasticity modeling, that graphene nanoribbons can, indeed, be transformed into carbon nanotubes by means of twisting. The chiralities of the tubes thus fabricated can be not only predicted but also externally controlled. This twisting route is an opportunity for nanofabrication, and is easily generalizable to ribbons made of other planar nanomaterials.

http://prb.aps.org/abstract/PRB/v85/i8/e085428

Febr. 9. - Febr. 16. (2012)

Válogatta: Tóvári Endre

Nonlinear detection of spin currents in graphene with non-magnetic electrodes

Ivan J. Vera-Marun,Vishal Ranjan & Bart J. van Wees doi:10.1038/nphys2219

Published online: 12 February 2012

The abilities to inject and detect spin carriers are fundamental for research on transport and manipulation of spin information1, 2. Pure electronic spin currents have been recently studied in nanoscale electronic devices using a non-local lateral geometry, both in metallic systems 3 and in semiconductors4. To unlock the full potential of spintronics we must understand the interactions of spin with other degrees of freedom. Such interactions have been explored recently, for example, by using spin Hall5, 6, 7 or spin thermoelectric effects6, 8,9. Here we present the detection of non-local spin signals using non-magnetic detectors, through an as-yet-unexplored nonlinear interaction between spin and charge. In analogy to the Seebeck effect10, where a heat current generates a charge potential, we demonstrate that a spin current in a paramagnet leads to a charge potential, if the conductivity is energy dependent. We use graphene11 as a model system to study this effect, as recently proposed12. The physical concept demonstrated here is generally valid, opening new possibilities for spintronics.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2219.html

Long Spin Relaxation Times in Wafer Scale Epitaxial Graphene on SiC(0001)

Thomas Maassen*†, J. Jasper van den Berg†, Natasja IJbema†, Felix Fromm‡, Thomas Seyller‡, Rositza Yakimova§, and Bart J. van Wees†

Nano Lett., Article ASAP

DOI: 10.1021/nl2042497

Publication Date (Web): February 10, 2012

We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times τS in monolayer graphene, while the spin diffusion coefficient DS is strongly reduced compared to typical results on exfoliated graphene. The increase of τS is probably related to the changed substrate, while the cause for the small value of DS remains an open question.

http://pubs.acs.org/doi/abs/10.1021/nl2042497

Remote Catalyzation for Direct Formation of Graphene Layers on Oxides

Po-Yuan Teng†, Chun-Chieh Lu†, Kotone Akiyama-Hasegawa‡, Yung-Chang Lin†, Chao-Hui Yeh†, Kazu Suenaga‡, and Po-Wen Chiu*†

DOI: 10.1021/nl204024k

Publication Date (Web): February 14, 2012

Direct deposition of high-quality graphene layers on insulating substrates such as SiO2 paves the way toward the development of graphene-based high-speed electronics. Here, we describe a novel growth technique that enables the direct deposition of graphene layers on SiO2 with crystalline quality potentially comparable to graphene grown on Cu foils using chemical vapor deposition (CVD). Rather than using Cu foils as substrates, our approach uses them to provide subliming Cu atoms in the CVD process. The prime feature of the proposed technique is remote catalyzation using floating Cu and H atoms for the decomposition of hydrocarbons. This allows for the direct graphitization of carbon radicals on oxide surfaces, forming isolated low-defect graphene layers without the need for postgrowth etching or evaporation of the metal catalyst. The defect density of the resulting graphene layers can be significantly reduced by tuning growth parameters such as the gas ratios, Cu surface areas, and substrate-to-Cu distance. Under optimized conditions, graphene layers with nondiscernible Raman D peaks can be obtained when predeposited graphite flakes are used as seeds for extended growth.

http://pubs.acs.org/doi/abs/10.1021/nl204024k

Van der Waals Epitaxy of InAs Nanowires Vertically Aligned on Single-Layer Graphene

Young Joon Hong*†, Wi Hyoung Lee‡, Yaping Wu‡, Rodney S. Ruoff*‡, and Takashi Fukui*†

DOI: 10.1021/nl204109t

Publication Date (Web): February 10, 2012

Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal–organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.

http://pubs.acs.org/doi/abs/10.1021/nl204109t

Ambipolar MoS2 Thin Flake Transistors

Yijin Zhang†, Jianting Ye*†, Yusuke Matsuhashi†, and Yoshihiro Iwasa*†‡

DOI: 10.1021/nl2021575

Publication Date (Web): January 25, 2012

Field effect transistors (FETs) made of thin flake single crystals isolated from layered materials have attracted growing interest since the success of graphene. Here, we report the fabrication of an electric double layer transistor (EDLT, a FET gated by ionic liquids) using a thin flake of MoS2, a member of the transition metal dichalcogenides, an archetypal layered material. The EDLT of the thin flake MoS2 unambiguously displayed ambipolar operation, in contrast to its commonly known bulk property as an n-type semiconductor. High-performance transistor operation characterized by a large “ON” state conductivity in the order of mS and a high on/off ratio >102 was realized for both hole and electron transport. Hall effect measurements revealed mobility of 44 and 86 cm2 V–1 s–1 for electron and hole, respectively. The hole mobility is twice the value of the electron mobility, and the density of accumulated carrier reached 1 × 1014 cm–2, which is 1 order of magnitude larger than conventional FETs with solid dielectrics. The high-density carriers of both holes and electrons can create metallic transport in the MoS2 channel. The present result is not only important for device applications with new functionalities, but the method itself would also act as a protocol to study this class of material for a broader scope of possibilities in accessing their unexplored properties.

http://pubs.acs.org/doi/abs/10.1021/nl2021575

Three-Terminal Graphene Negative Differential Resistance Devices

Yanqing Wu, Damon B. Farmer*, Wenjuan Zhu, Shu-Jen Han, Christos D. Dimitrakopoulos, Ageeth A. Bol, Phaedon Avouris*, and Yu-Ming Lin*

IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, United States

DOI: 10.1021/nn205106z

Publication Date (Web): February 10, 2012

A new mechanism for negative differential resistance (NDR) is discovered in three-terminal graphene devices based on a field-effect transistor configuration. This NDR effect is a universal phenomenon for graphene and is demonstrated in devices fabricated with different types of graphene materials and gate dielectrics. Operation of conventional NDR devices is usually based on quantum tunneling or intervalley carrier transfer, whereas the NDR behavior observed here is unique to the ambipolar behavior of zero-bandgap graphene and is associated with the competition between electron and hole conduction as the drain bias increases. These three terminal graphene NDR devices offer more operation flexibility than conventional two-terminal devices based on tunnel diodes, Gunn diodes, or molecular devices, and open up new opportunities for graphene in microwave to terahertz applications.

http://pubs.acs.org/doi/abs/10.1021/nn205106z

Spin-selective transport through helical molecular systems

R. Gutierrez1, E. Díaz1,2, R. Naaman3, and G. Cuniberti1,4

Received 1 February 2012; published 16 February 2012

Highly spin-selective transport of electrons through a helically shaped electrostatic potential is demonstrated in the frame of a minimal model approach. The effect is significant even for weak spin-orbit coupling. Two main factors determine the selectivity: an unconventional Rashba-like spin-orbit interaction, reflecting the helical symmetry of the system, and a weakly dispersive electronic band of the helical system. The weak electronic coupling, associated with the small dispersion, leads to a low mobility of the charges in the system and allows even weak spin-orbit interactions to be effective. The results are expected to be generic for chiral molecular systems displaying low spin-orbit coupling and low conductivity.

DOI:10.1103/PhysRevB.85.081404

http://prb.aps.org/abstract/PRB/v85/i8/e081404

Signatures of Majorana Fermions in Hybrid Superconductor-Topological Insulator Devices

J. R. Williams, A. J. Bestwick, P. Gallagher, Seung Sae Hong, Y. Cui, Andrew S. Bleich, J. G. Analytis, I. R. Fisher, D. Goldhaber-Gordon

(Submitted on 10 Feb 2012)

The ability to measure and manipulate complex particles in the solid state is a cornerstone of modern condensed-matter physics. Typical excitations of a sea of electrons, called quasiparticles, have properties similar to those of free electrons. However, in recent years exotic excitations with very different properties have been created in designer quantum materials, including Dirac fermions in graphene [1] and fractionally charged quasiparticles in fractional quantum Hall systems [2]. Here we report signatures of a new quasiparticle -- the Majorana fermion -- in Josephson junctions consisting of two superconducting leads coupled through a three-dimensional topological insulator [3]. We observe two striking departures from the common transport properties of Josephson junctions: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model in which a one-dimensional wire of Majorana fermions is present along the width of the junction, similar to a theoretical prediction by Fu and Kane [4]. These results present an opening into the investigation of Majorana fermions in the solid state and their exotic properties, including non-Abelian statistics [5], a suggested basis for fault-tolerant quantum computation [6].

http://arxiv.org/abs/1202.2323

Non-linear resistivity and heat dissipation in monolayer graphene

A. S. Price, S. M. Hornett, A. V. Shytov, E. Hendry, D. W. Horsell

(Submitted on 15 Feb 2012)

We have experimentally studied the nonlinear nature of electrical conduction in monolayer graphene devices on silica substrates. This nonlinearity manifests itself as a nonmonotonic dependence of the differential resistance on applied DC voltage bias across the sample. At temperatures below ~70K, the differential resistance exhibits a peak near zero bias that can be attributed to self-heating of the charge carriers. We show that the shape of this peak arises from a combination of different energy dissipation mechanisms of the carriers. The energy dissipation at higher carrier temperatures depends critically on the length of the sample. For samples longer than 10um the heat loss is shown to be determined by optical phonons at the silica-graphene interface.

http://xxx.lanl.gov/abs/1202.3394

The Integration of High-k Dielectric on Two-Dimensional Crystals by Atomic Layer Deposition

Han Liu, Kun Xu, Xujie Zhang, Peide D. Ye

(Submitted on 15 Feb 2012)

We investigate the integration of Al2O3 high-k dielectric on two-dimensional (2D) crystals of boron nitride (BN) and molybdenum disulfide (MoS2) by atomic layer deposition (ALD). We demonstrate the feasibility of direct ALD growth with trimethylaluminum(TMA) and water as precursors on both 2D crystals. Through theoretical and experimental studies, we found that the initial ALD cycles play the critical role, during which physical adsorption dominates precursor adsorption at the semiconductor surface. We model the initial ALD growth stages at the 2D surface by analyzing Lennard-Jones Potentials, which could guide future optimization of the ALD process on 2D crystals.

http://xxx.lanl.gov/abs/1202.3391

Precision comparison of the quantum Hall effect in graphene and gallium arsenide

T.J.B.M. Janssen, J.M. Williams, N.E. Fletcher, R. Goebel, A. Tzalenchuk, R. Yakimova, S. Lara-Avila, S. Kubatkin, V.I. Fal'ko

(Submitted on 14 Feb 2012)

The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantised resistance values within the relative standard uncertainty of our measurement of $8.7\times 10^{-11}$. The result places new tighter limits on any possible correction terms to the simple relation $R_{\rm K}=h/e^2$, and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterisation of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget. http://xxx.lanl.gov/abs/1202.2985

Graphene radio: Detecting radiowaves with a single atom sheet

Mircea Dragoman, Dan Neculoiu, Alina Cismaru, George Deligeorgis, George Konstantinidis, Daniela Dragoman

(Submitted on 9 Feb 2012)

We present the experimental evidence of RF demodulation by a graphene monolayer embedded in a coplanar structure. The demodulator was tested in the frequency range from 100 MHz to 25 GHz using amplitude modulated input signals. An input power of 0 dBm (1 mW) was used which is the typical power emitted for short range wireless communication systems, such as Bluetooth. The graphene demodulator exhibits good signal response in the frequency range associated to industrial, scientific and medical (ISM) radio band (2.4 GHz).

http://xxx.lanl.gov/abs/1202.1968

Febr. 2. - Febr. 9. (2012)

Válogatta: Tóvári Endre

Shallow pockets and very strong coupling superconductivity in FeSe_xTe_1−x

Y. Lubashevsky, E. Lahoud, K. Chashka, D. Podolsky, A. Kanigel

The celebrated Bardeen–Cooper–Schrieffer (BCS) theory has been successful in explaining metallic superconductors, yet many believe that it must be modified to deal with the newer high-temperature superconductors. A possible extension is provided by the BCS–Bose–Einstein condensate (BEC) theory, describing a smooth evolution from a system of weakly interacting pairs to a BEC of molecules of strongly bound fermions. Despite its appeal, spectroscopic evidence for the BCS–BEC crossover has never been observed in solids. Here we report electronic structure measurements in FeSe_xTe_1−x showing that these materials are in the BCS–BEC crossover regime. Above the superconducting transition temperature, T_c, we find multiple bands with remarkably small values for the Fermi energy ε_F. Yet, in the superconducting state, the gap Δ is comparable to ε_F. The ratio Δ/ε_F≈0.5 is much larger than found in any previously studied superconductor, resulting in an anomalous dispersion of the coherence peak very similar to that found in cold Fermi gas experiments, in agreement with the predictions of the BCS–BEC crossover theory.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2216.html

Extremely Bendable, High-Performance Integrated Circuits Using Semiconducting Carbon Nanotube Networks for Digital, Analog, and Radio-Frequency Applications

Chuan Wang, Jun-Chau Chien, Kuniharu Takei, Toshitake Takahashi, Junghyo Nah, Ali M. Niknejad, and Ali Javey

Solution-processed thin-films of semiconducting carbon nanotubes as the channel material for flexible electronics simultaneously offers high performance, low cost, and ambient stability, which significantly outruns the organic semiconductor materials. In this work, we report the use of semiconductor-enriched carbon nanotubes for high-performance integrated circuits on mechanically flexible substrates for digital, analog and radio frequency applications. The as-obtained thin-film transistors (TFTs) exhibit highly uniform device performance with on-current and transconductance up to 15 μA/μm and 4 μS/μm. By performing capacitance–voltage measurements, the gate capacitance of the nanotube TFT is precisely extracted and the corresponding peak effective device mobility is evaluated to be around 50 cm²V^–1s^–1. Using such devices, digital logic gates including inverters, NAND, and NOR gates with superior bending stability have been demonstrated. Moreover, radio frequency measurements show that cutoff frequency of 170 MHz can be achieved in devices with a relatively long channel length of 4 μm, which is sufficient for certain wireless communication applications. This proof-of-concept demonstration indicates that our platform can serve as a foundation for scalable, low-cost, high-performance flexible electronics.

http://pubs.acs.org/doi/full/10.1021/nl2043375

Magnetoelectric Charge Trap Memory

Uwe Bauer, Marek Przybylski, Jürgen Kirschner, and Geoffrey S. D. Beach

It is demonstrated that a charge-trapping layer placed in proximity to a ferromagnetic metal enables efficient electrical and optical control of the metal’s magnetic properties. Retention of charge trapped inside the charge-trapping layer provides nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. As such, an engineered charge-trapping layer can be used to realize the magnetoelectric equivalent to today’s pervasive charge trap flash memory technology. Moreover, by supplying trapped charges optically instead of electrically, a focused laser beam can be used to imprint the magnetic state into a continuous metal film.

http://pubs.acs.org/doi/full/10.1021/nl204114t

Number of observable features in the acoustic Raman spectra of nanocrystals

Lucien Saviot, Nicolas Combe, and Adnen Mlayah

Low-frequency Raman-scattering spectra are presented for gold nanocrystals with diameters 3.5 and 13 nm. The frequencies of the Raman peaks but also their number are shown to vary with the nanocrystal size. These results are analyzed using both the continuous elastic medium approximation and an atomistic approach. We show that the number of atoms in the nanocrystal determines an upper limit of the number of observable Raman features. The frequency range in which the continuous elastic medium approximation is valid is defined by comparison with the calculations based on the atomistic approach.

http://prb.aps.org/abstract/PRB/v85/i7/e075405

Electric Field Confinement Effect on Charge Transport in Organic Field-Effect Transistors

Xiaoran Li, Andrey Kadashchuk, Ivan I. Fishchuk, Wiljan T. T. Smaal, Gerwin Gelinck, Dirk J. Broer, Jan Genoe, Paul Heremans, and Heinz Bässler

While it is known that the charge-carrier mobility in organic semiconductors is only weakly dependent on the electric field at low fields, the experimental mobility in organic field-effect transistors using silylethynyl-substituted pentacene is found to be surprisingly field dependent at low source-drain fields. Corroborated by scanning Kelvin probe measurements, we explain this observation by the severe difference between local conductivities within grains and at grain boundaries. Redistribution of accumulated charges creates very strong local lateral fields in the latter regions. We further confirm this picture by verifying that the charge mobility in channels having no grain boundaries, made from the same organic semiconductor, is not significantly field dependent. We show that our model allows us to quantitatively model the source-drain field dependence of the mobility in polycrystalline organic transistors.

http://prl.aps.org/abstract/PRL/v108/i6/e066601

Excitation of collective modes in a quantum flute

Kristinn Torfason, Andrei Manolescu, Valeriu Molodoveanu, Vidar Gudmundsson

We use a generalized master equation (GME) formalism to describe the non-equilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials which simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum "flute" device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.

http://xxx.lanl.gov/abs/1202.0566

Atomically thin boron nitride: a tunnelling barrier for graphene devices

Liam Britnell, Roman V. Gorbachev, Rashid Jalil, Branson D. Belle, Fred Schedin, Mikhail I. Katsnelson, Laurence Eaves, Sergey V. Morozov, Alexander S. Mayorov, Nuno M. R. Peres, Antonio H. Castro Neto, Jon Leist, Andre K. Geim, Leonid A. Ponomarenko, Kostya S. Novoselov

We investigate the electronic properties of heterostructures based on ultrathin hexagonal boron nitride (h-BN) crystalline layers sandwiched between two layers of graphene as well as other conducting materials (graphite, gold). The tunnel conductance depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Exponential behaviour of I-V characteristics for graphene/BN/graphene and graphite/BN/graphite devices is determined mainly by the changes in the density of states with bias voltage in the electrodes. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field; it offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.

http://xxx.lanl.gov/abs/1202.0735

Gate Defined Quantum Confinement in Suspended Bilayer Graphene

Monica T. Allen, Jens Martin, Amir Yacoby

Quantum confined devices that manipulate single electrons in graphene are emerging as attractive candidates for nanoelectronics applications. Previous experiments have employed etched graphene nanostructures, but edge and substrate disorder severely limit device functionality. Here we present a technique that builds quantum confined structures in suspended bilayer graphene with tunnel barriers defined by external electric fields that break layer inversion symmetry, thereby eliminating both edge and substrate disorder. We report clean quantum dot formation in two regimes: at zero magnetic field B using the single particle energy gap induced by a perpendicular electric field and at B > 0 using the quantum Hall ferromagnet ν = 0 gap for confinement. Coulomb blockade oscillations exhibit periodicity consistent with electrostatic simulations based on local top gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates single electron transport with high device quality and access to vibrational modes, enabling broad applications from electromechanical sensors to quantum bits.

http://xxx.lanl.gov/abs/1202.0820

Transport through side-coupled double quantum dots: from weak to strong interdot coupling

D. Y. Baines, T. Meunier, D. Mailly, A. D. Wieck, C. Bäuerle, L. Saminadayar, Pablo S. Cornaglia, Gonzalo Usaj, C. A. Balseiro, D. Feinberg

We report low-temperature transport measurements through a double quantum dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak coupling regime can be understood by considering a single level on each dot, in the intermediate and strong coupling regimes, the multi-level nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong and weak coupling regimes, the double quantum dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate coupling regime the device presents a single dot-like molecular behavior as the molecular wavefunctions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed.

http://xxx.lanl.gov/abs/1202.1580

Giant negative magnetoresistance in high-mobility two-dimensional electron systems

A. T. Hatke, M. A. Zudov, J. L. Reno, L. N. Pfeiffer, and K. W. West

We report on a giant negative magnetoresistance in very high mobility GaAs/AlGaAs heterostructures and quantum wells. The effect is the strongest at B≃1 kG, where the magnetoresistivity develops a minimum emerging at T≲2 K. Unlike the zero-field resistivity which saturates at T≃2 K, the resistivity at this minimum continues to drop at an accelerated rate to much lower temperatures and becomes several times smaller than the zero-field resistivity. Unexpectedly, we also find that the effect is destroyed not only by increasing temperature but also by modest in-plane magnetic fields. The analysis shows that giant negative magnetoresistance cannot be explained by existing theories considering interaction-induced or disorder-induced corrections.

http://prb.aps.org/abstract/PRB/v85/i8/e081304

Resonance-hybrid states in a triple quantum dot

S. Amaha, T. Hatano, H. Tamura, S. Teraoka, T. Kubo, Y. Tokura, D. G. Austing, and S. Tarucha

Delocalization by resonance between contributing structures explains the enhanced stability of resonance-hybrid molecules. Here we report the realization of resonance-hybrid states in a few-electron triple quantum dot (TQD) obseved by excitation spectroscopy. The stabilization of the resonance-hybrid state and the bond between contributing states are achieved through access to the intermediate states with double occupancy of the dots. This explains why the energy of the hybridized singlet state is signiﬁcantly lower than that of the triplet state. The properties of the three-electron doublet states can also be understood with the resonance-hybrid picture and geometrical phase. As well as for fundamental TQD physics, our results are useful for the investigation of materials such as quantum dot arrays, quantum information processors, and chemical reaction and quantum simulators.

http://prb.aps.org/pdf/PRB/v85/i8/e081301

Jan. 25. - Feb. 1. (2012)

Válogatta: Balogh Zoltán

A local optical probe for measuring motion and stress in a nanoelectromechanical system

Antoine Reserbat-Plantey, Lae¨titia Marty, Olivier Arcizet, Nedjma Bendiab and Vincent Bouchiat

Nanoelectromechanical systems can be operated as ultrasensitive mass sensors and ultrahigh-frequency resonators, and can also be used to explore fundamental physical phenomena such as nonlinear damping and quantum effects in macroscopic objects. Various dissipation mechanisms are known to limit the mechanical quality factors of nanoelectromechanical systems and to induce aging due to material degradation, so there is a need for methods that can probe the motion of these systems, and the stresses within them, at the nanoscale. Here, we report a non-invasive local optical probe for the quantitative measurement of motion and stress within a nanoelectromechanical system, based on Fizeau interferometry and Raman spectroscopy. The system consists of a multilayer graphene resonator that is clamped to a gold film on an oxidized silicon surface. The resonator and the surface both act as mirrors and therefore define an optical cavity. Fizeau interferometry provides a calibrated measurement of the motion of the resonator, while Raman spectroscopy can probe the strain within the system and allows a purely spectral detection of mechanical resonance at the nanoscale.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.250.html

How Does a Single Pt Nanocatalyst Behave in Two Different Reactions? A Single-Molecule Study

Kyu Sung Han, Guokun Liu, Xiaochun Zhou, Rita E. Medina, and Peng Chen

Using single-molecule microscopy of fluorogenic reactions we studied Pt nanoparticle catalysis at single-particle, single-turnover resolution for two reactions: one an oxidative N-deacetylation and the other a reductive N-deoxygenation. These Pt nanoparticles show distinct catalytic kinetics in these two reactions: one following noncompetitive reactant adsorption and the other following competitive reactant adsorption. In both reactions, single nanoparticles exhibit temporal activity fluctuations attributable to dominantly spontaneous surface restructuring. Depending on the reaction sequence, single Pt nanoparticles may or may not show activity correlations in catalyzing both reactions, reflecting the structure insensitivity of the N-deacetylation reaction and the structure sensitivity of the N-deoxygenation reaction.

http://pubs.acs.org/doi/abs/10.1021/nl203677b

Single-Molecule Force-Clamp Experiments Reveal Kinetics of Mechanically Activated Silyl Ester Hydrolysis

Sebastian W. Schmidt, Pavel Filippov, Alfred Kersch, Martin K. Beyer, and Hauke Clausen-Schaumann

We have investigated the strength of silyl ester bonds formed between carboxymethylated amylose (CMA) molecules and silane-functionalized silicon oxide surfaces using AFM-based single-molecule force spectroscopy in the force-clamp mode. Single tethered CMA molecules were picked up, and bond lifetimes were determined at constant clamp forces of 0.8, 1.0, and 1.2 nN at seven temperatures between 295 and 320 K at pH 2.0. The results reveal biexponential rupture kinetics. To obtain the reaction rate constants for each force and temperature individually, the results were analyzed with a biexponential kinetic model using the maximum likelihood estimation (MLE) method. The force-independent kinetic and structural parameters of the underlying bond rupture mechanisms were extracted by fitting the entire data set with a parallel MLE fit procedure using the Zhurkov/Bell model and, alternatively, an Arrhenius kinetics model combined with a Morse potential as an analytic representation of the binding potential. With activation energies between 37 and 40 kJ mol–1, and with Arrhenius prefactors between 5 × 104 and 2 × 106 s–1, the results point to the hydrolysis of the silyl ester bond.

http://pubs.acs.org/doi/abs/10.1021/nn204111w

A surface-anchored molecular four-level conductance switch based on single proton transfer

Willi Auwa¨rter, Knud Seufert, Felix Bischoff, David Ecija, Saranyan Vijayaraghavan, Sushobhan Joshi, Florian Klappenberger, Niveditha Samudrala and Johannes V. Barth

The development of a variety of nanoscale applications requires the fabrication and control of atomic or molecular switches that can be reversibly operated by light, a shortrange force electric current or other external stimuli. For such molecules to be used as electronic components, they should be directly coupled to a metallic support and the switching unit should be easily connected to other molecular species without suppressing switching performance. Here, we show that a free-base tetraphenyl-porphyrin molecule, which is anchored to a silver surface, can function as a molecular conductance switch. The saddle-shaped molecule has two hydrogen atoms in its inner cavity that can be flipped between two states with different local conductance levels using the electron current through the tip of a scanning tunnelling microscope. Moreover, by deliberately removing one of the hydrogens, a four-level conductance switch can be created. The resulting device, which could be controllably integrated into the surrounding nanoscale environment, relies on the transfer of a single proton and therefore contains the smallest possible atomistic switching unit.

http://www.nature.com/nnano/journal/v7/n1/full/nnano.2011.211.html

Transport spectroscopy of symmetry-broken insulating states in bilayer graphene

J. Velasco Jr, L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, C. N. Lau, C. Varma, R. Stillwell, D. Smirnov, Fan Zhang, J. Jung & A. H. MacDonald

Bilayer graphene is an attractive platform for studying new two-dimensional electron physics, because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron–electron interaction effects. Theory predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps are analogous to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects accompanied by spontaneous quantum Hall effects. Although recent experiments have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source–drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ~15 mV nm−1, but it increases monotonically with magnetic field, with an apparent particle–hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.251.html

Plasmonic Systems Unveiled by Fano Resonances

Yan Francescato, Vincenzo Giannini, and Stefan A. Maier

We show in detail how a derivation of Fano theory can serve as a new paradigm to study, understand, and control the interaction of nano-objects with light. Examples include a plasmonic crystal, a dolmen-type structure sustaining dark and bright plasmon modes, and a nanoshell heptamer. On the basis of only three coupling factors, a straightforward analytical formula is obtained, only assuming a plasmonic resonance for the continuum, and retaining the nonclassical character of the original formalism. It allows one to predict, reproduce, or decompose Fano interferences solely in terms of the physical properties of the uncoupled nanostructures when available, without the need of additional fitting parameters.

http://pubs.acs.org/doi/abs/10.1021/nn2050533

Electromagnetic Energy Transport in Nanoparticle Chains via Dark Plasmon Modes

David Solis, Jr., Britain Willingham, Scott L. Nauert, Liane S. Slaughter, Jana Olson, Pattanawit Swanglap, Aniruddha Paul, Wei-Shun Chang, and Stephan Link

Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows one to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. We show electromagnetic energy transport with gold nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calculations confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.

http://pubs.acs.org/doi/abs/10.1021/nl2039327

Flexible Gigahertz Transistors Derived from Solution-Based Single-Layer Graphene

Cédric Sire, Florence Ardiaca, Sylvie Lepilliet, Jung-Woo T. Seo, Mark C. Hersam, Gilles Dambrine, Henri Happy, and Vincent Derycke

Flexible electronics mostly relies on organic semiconductors but the limited carrier velocity in polymers and molecular films prevents their use at frequencies above a few megahertz. Conversely, the high potential of graphene for high-frequency electronics on rigid substrates was recently demonstrated. We conducted the first study of solution-based graphene transistors at gigahertz frequencies, and we show that solution-based single-layer graphene ideally combines the required properties to achieve high speed flexible electronics on plastic substrates. Our graphene flexible transistors have current gain cutoff frequencies of 2.2 GHz and power gain cutoff frequencies of 550 MHz. Radio frequency measurements directly performed on bent samples show remarkable mechanical stability of these devices and demonstrate the advantages of solution-based graphene field-effect transistors over other types of flexible transistors based on organic materials.

http://pubs.acs.org/doi/abs/10.1021/nl203316r

Atomically localized plasmon enhancement in monolayer graphene

Wu Zhou, Jaekwang Lee, Jagjit Nanda, Sokrates T. Pantelides, Stephen J. Pennycook & Juan-Carlos Idrobo

Plasmons in graphene can be tuned by using electrostatic gating or chemical doping, and the ability to confine plasmons in very small regions could have applications in optoelectronics, plasmonics and transformation optics12. However, little is known about how atomic-scale defects influence the plasmonic properties of graphene. Moreover, the smallest localized plasmon resonance observed in any material to date has been limited to around 10 nm. Here, we show that surface plasmon resonances in graphene can be enhanced locally at the atomic scale. Using electron energy-loss spectrum imaging in an aberration-corrected scanning transmission electron microscope, we find that a single point defect can act as an atomic antenna in the petahertz (1015 Hz) frequency range, leading to surface plasmon resonances at the subnanometre scale.

http://www.nature.com/nnano/journal/vaop/ncurrent/abs/nnano.2011.252.html

Measurement of Quantum Noise in a Carbon Nanotube Quantum Dot in the Kondo Regime

J. Basset, A. Yu. Kasumov, C. P. Moca, G. Zaránd, P. Simon, H. Bouchiat, and R. Deblock

he current emission noise of a carbon nanotube quantum dot in the Kondo regime is measured at frequencies ν of the order or higher than the frequency associated with the Kondo effect kBTK/h, with TK the Kondo temperature. The carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise detector, a superconductor-insulator-superconductor junction. We find for hν≈kBTK a Kondo effect related singularity at a voltage bias eV≈hν, and a strong reduction of this singularity for hν≈3kBTK, in good agreement with theory. Our experiment constitutes a new original tool for the investigation of the nonequilibrium dynamics of many-body phenomena in nanoscale devices.

http://prl.aps.org/abstract/PRL/v108/i4/e046802

Complete Optical Absorption in Periodically Patterned Graphene

Sukosin Thongrattanasiri, Frank H. L. Koppens, and F. Javier García de Abajo

We demonstrate that 100% light absorption can take place in a single patterned sheet of doped graphene. General analysis shows that a planar array of small particles with losses exhibits full absorption under critical-coupling conditions provided the cross section of each individual particle is comparable to the area of the lattice unit cell. Specifically, arrays of doped graphene nanodisks display full absorption when supported on a substrate under total internal reflection and also when lying on a dielectric layer coating a metal. Our results are relevant for infrared light detectors and sources, which can be made tunable via electrostatic doping of graphene.

http://prl.aps.org/abstract/PRL/v108/i4/e047401

Nov. 17. - Nov. 23. (2011)

Válogatta: Oroszlány László

Nov. 17. - Nov. 23. (2011)

Válogatta: Csontos Miklós

Single-Electron Capacitance Spectroscopy of Individual Dopants in Silicon

M. Gasseller, M. DeNinno, R. Loo, J. F. Harrison, M. Caymax, S. Rogge and S. H. Tessmer

Motivated by recent transport experiments and proposed atomic-scale semiconductor devices, we present measurements that extend the reach of scanned-probe methods to discern the properties of individual dopants tens of nanometers below the surface of a silicon sample. Using a capacitance-based approach, we have both spatially resolved individual subsurface boron acceptors and detected spectroscopically single holes entering and leaving these minute systems of atoms. A resonance identified as the B+ state is shown to shift in energy from acceptor to acceptor. We examine this behavior with respect to nearestneighbor distances. By directlymeasuring the quantum levels and testing the effect of dopant-dopant interactions, this method represents a valuable tool for the development of future atomicscale semiconductor devices.

http://pubs.acs.org/doi/abs/10.1021/nl2025163

Coulomb Blockade in an Open Quantum Dot

S. Amasha, I. G. Rau, M. Grobis, R. M. Potok, H. Shtrikman and D. Goldhaber-Gordon

We report the observation of Coulomb blockade in a quantum dot contacted by two quantum point contacts each with a single fully transmitting mode, a system thought to be well described without invoking Coulomb interactions. Below 50 mK we observe a periodic oscillation in the conductance of the dot with gate voltage, corresponding to a residual quantization of charge. From the temperature and magnetic field dependence, we infer the oscillations are mesoscopic Coulomb blockade, a type of Coulomb blockade caused by electron interference in an otherwise open system.

http://prl.aps.org/abstract/PRL/v107/i21/e216804

Majorana fermions in a topological-insulator nanowire proximity-coupled to an s-wave superconductor

A. Cook and M. Franz

A finite-length topological-insulator nanowire, proximity-coupled to an ordinary bulk s-wave superconductor and subject to a longitudinal applied magnetic field, is shown to realize a one-dimensional topological superconductor with unpaired Majorana fermions localized at both ends. This situation occurs under a wide range of conditions and constitutes an easily accessible physical realization of the elusive Majorana particle in a solid-state system.

http://prb.aps.org/abstract/PRB/v84/i20/e201105

Josephson and Andreev transport through quantum dots

A. Martin-Rodero and A. Levy Yeyati

In this article we review the state of the art on the transport properties of quantum dot systems connected to superconducting and normal electrodes. The review is mainly focused on the theoretical achievements although a summary of the most relevant experimental results is also given. A large part of the discussion is devoted to the single level Anderson type models generalized to include superconductivity in the leads, which already contains most of the interesting physical phenomena. Particular attention is paid to the competition between pairing and Kondo correlations, the emergence of pi-junction behavior, the interplay of Andreev and resonant tunneling, and the important role of Andreev bound states which characterized the spectral properties of most of these systems. We give technical details on the several different analytical and numerical methods which have been developed for describing these properties. We further discuss the recent theoretical efforts devoted to extend this analysis to more complex situations like multidot, multilevel or multiterminal configurations in which novel phenomena is expected to emerge. These include control of the localized spin states by a Josephson current and also the possibility of creating entangled electron pairs by means of non-local Andreev processes.

http://xxx.lanl.gov/abs/1111.4939

Time scales in the dynamics of an interacting quantum dot

L. Debora Contreras-Pulido, Janine Splettstoesser, Michele Governale, Jurgen Konig and Markus Buttiker

We analyze the dynamics of a single-level quantum dot with Coulomb interaction, weakly tunnel coupled to an electronic reservoir, after it has been brought out of equilibrium, e.g. by a step-pulse potential. We investigate the exponential decay towards the equilibrium state, which is governed by three time scales. In addition to the charge and spin relaxation time there is a third time scale which is independent of the level position and the Coulomb interaction. This time scale emerges in the time evolution of physical quantities sensitive to two-particle processes.

http://xxx.lanl.gov/abs/1111.4135

Spin-half paramagnetism in graphene induced by point defects

R. R. Nair, M. Sepioni, I-Ling Tsai, O. Lehtinen, J. Keinonen, A. V. Krasheninnikov, T. Thomson, A. K. Geim and I. V. Grigorieva

Using magnetization measurements, we show that point defects in graphene – fluorine adatoms and irradiation defects (vacancies) – carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene’s low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene’s structural stability.

http://xxx.lanl.gov/abs/1111.3775

Majorana Fermions in Semiconductor Nanowires

Tudor D. Stanescu, Roman M. Lutchyn and S. Das Sarma

We study multiband semiconducting nanowires proximity-coupled with an s-wave superconductor and calculate the topological phase diagram as a function of the chemical potential and magnetic field. The non-trivial topological state corresponds to a superconducting phase supporting an odd number of pairs of Majorana modes localized at the ends of the wire, whereas the non-topological state corresponds to a superconducting phase with no Majoranas or with an even number of pairs of Majorana modes. Our key finding is that multiband occupancy not only lifts the stringent constraint of one-dimensionality, but also allows having higher carrier density in the nanowire. Consequently, multiband nanowires are better-suited for stabilizing the topological superconducting phase and for observing the Majorana physics. We present a detailed study of the parameter space for multiband semiconductor nanowires focusing on understanding the key experimental conditions required for the realization and detection of Majorana fermions in solid-state systems. We include various sources of disorder and characterize their effects on the stability of the topological phase. Finally, we calculate the local density of states as well as the differential tunneling conductance as functions of external parameters and predict the experimental signatures that would establish the existence of emergent Majorana zero-energy modes in solid-state systems.

http://xxx.lanl.gov/abs/1106.3078

Nov. 3. - Nov. 10. (2011)

Válogatta: Makk Péter

Spatially resolved electronic inhomogeneities of graphene due to subsurface charges

Andres Castellanos-Gomez, Roel H. M. Smit, Nicolás Agraït, Gabino Rubio-Bollinger

We probe the local inhomogeneities in the electronic properties of exfoliated graphene due to the presence of charged impurities in the SiO2 substrate using a combined scanning tunneling and electrostatic force microscope. Contact potential difference measurements using electrostatic force microscopy permit us to obtain the average charge density but it does not provide enough resolution to identify individual charges. We find that the tunneling current decay constant, which is related to the local tunneling barrier height, enables one to probe the electronic properties of graphene distorted at the nanometer scale by individual charged impurities. We observe that such inhomogeneities do not show long range ordering and their surface density obtained by direct counting is consistent with the value obtained by macroscopic charge density measurements. These microscopic perturbations of the carrier density significantly alter the electronic properties of graphene, and their characterization is essential for improving the performance of graphene based devices.

http://arxiv.org/abs/1111.0840

Switching and Rectification of a Single Light-sensitive Diarylethene Molecule Sandwiched between Graphene Nanoribbons

Yongqing Cai, Aihua Zhang, Yuan Ping Feng, Chun Zhang

The 'open' and 'closed' isomers of the diarylethene molecule that can be converted between each other upon photo-excitation are found to have drastically different current-voltage characteristics when sandwiched between two graphene nanoribbons (GNRs). More importantly, when one GNR is metallic and another one is semiconducting, strong rectification behavior of the 'closed' diarylethene isomer with the rectification ratio >10^3 is observed. The surprisingly high rectification ratio originates from the band gap of GNR and the bias-dependent variation of the lowest unoccupied molecular orbital (LUMO) of the diarylethene molecule, the combination of which completely shuts off the current at positive biases. Results presented in this paper may form the basis for a new class of molecular electronic devices.

http://arxiv.org/abs/1111.1811

Majorana fermions in superconducting nanowires without spin-orbit coupling

Authors: Morten Kjærgaard, Konrad Wölms, Karsten Flensberg

We show that confined Majorana fermions can exist in nanowires with proximity induced s-wave superconducting pairing if the direction of an external magnetic field rotates along the wire. The system is equivalent to nanowires with Rashba-type spin-orbit coupling, with strength proportional to the derivative of the field angle. For realistic parameters, we demonstrate that a set of permanent magnets can bring a nearby nanowire into the topologically non-trivial phase with localized Majorana modes at its ends. Without the requirement of spin-orbit coupling this opens up for a new route for demonstration and design of Majorana fermion systems.

http://arxiv.org/abs/1111.2129

Role of Polytetrahedral Structures in the Elongation and Rupture of Gold Nanowires

Christopher R. Iacovella†, William R. French†, Brandon G. Cook‡, Paul R. C. Kent§, and Peter T. Cummings†§*

We report comprehensive high-accuracy molecular dynamics simulations using the ReaxFF force field to explore the structural changes that occur as Au nanowires are elongated, establishing trends as a function of both temperature and nanowire diameter. Our simulations and subsequent quantitative structural analysis reveal that polytetrahedral structures (e.g., icosahedra) form within the “amorphous” neck regions, most prominently for systems with small diameter at high temperature. We demonstrate that the formation of polytetrahedra diminishes the conductance quantization as compared to systems without this structural motif. We demonstrate that use of the ReaxFF force field, fitted to high-accuracy first-principles calculations of Au, combines the accuracy of quantum calculations with the speed of semiempirical methods.

http://pubs.acs.org/doi/abs/10.1021/nn203941r

Spin Relaxation in InGaN Quantum Disks in GaN Nanowires

Animesh Banerjee†, Fatih Doğan‡, Junseok Heo†, Aurelien Manchon‡, Wei Guo†, and Pallab Bhattacharya

The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of 100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott–Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D’yakonov–Perel’ relaxation mechanism in the presence of Rashba spin–orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.

http://pubs.acs.org/doi/abs/10.1021/nl203091f

Electronic transport through single noble gas atoms

L. A. Zotti1, M. Bürkle2, Y. J. Dappe3, F. Pauly2, and J. C. Cuevas1

We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold electrodes. This explains the observations reported in metallic atomic-size contacts with adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the conductance because of the additional current path provided by their valence p states.

http://prb.aps.org/abstract/PRB/v84/i19/e193404

Andreev nanoprobe of half-metallic CrO2 films using superconducting cuprate tips

C. S. Turel, I. J. Guilaran, P. Xiong, and J. Y. T. Wei

Superconducting tips of YBa2Cu3O7−x were used to perform point-contact Andreev reflection spectroscopy on half-metallic CrO2 thin films. At 4.2 K, strong suppression of the d-wave Andreev reflection characteristics was observed, consistent with the high spin polarization of CrO2. Our technique was validated by comparison with data taken on non-magnetic Au films and with data taken by superconducting Pb tips. The point contacts were estimated to be ≲10 nm in size, attesting to their ballistic and microscopic nature. Our results demonstrate the feasibility of using superconducting cuprate tips as spin-sensitive nanoprobes of ferromagnets.

http://apl.aip.org/resource/1/applab/v99/i19/p192508_s1

Spin-filter Josephson junctions

Kartik Senapati,Mark G. Blamire & Zoe H. Barber

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years1. Of particular interest has been the realization of so called π-junctions with a built-in phase difference2, and induced triplet pairing3, 4. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (IF) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation5, 6 and an unconventional temperature dependence7 of the critical current Ic, difficulties with the few known IF materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of Icis strongly modified by the IF. In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the IF barrier can be modified by magnetic inhomogeneity in the barrier.

http://www.nature.com/nmat/journal/v10/n11/pdf/nmat3116.pdf

Fast control of nuclear spin polarization in an optically pumped single quantum dot

M. N. Makhonin, K. V. Kavokin, P. Senellart, A. Lemaître,A. J. Ramsay, M. S. Skolnick & A. I. Tartakovskii

Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or up to a few hundred mT for the hole spin13, 14. Recently this has been recognized as a resource for intrinsic control of quantum-dot-based spin quantum bits. However, only static long-lived Overhauser fields could be used10, 11. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 105 optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame15 leading to sub-μK nuclear spin temperatures, rapid adiabatic passage15, and spin squeezing16.

http://www.nature.com/nmat/journal/v10/n11/pdf/nmat3102.pdf

Okt. 28. - Nov. 3. (2011)

Válogatta: Márton Attila

Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires

D. Laroche, G. Gervais, M. P. Lilly& J. L. Reno DOI:10.1038/nnano.2011.182

Received 03 August 2011; Accepted 23 September 2011; Published online 30 October 2011 Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or use reduced-dimensional conductors1 such as carbon nanotubes2, 3, 4, 5, 6, nanowires7, 8, 9,10 and gated high-mobility two-dimensional electron systems11, 12, 13. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomena such as Coulomb drag, where a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have demonstrated Coulomb electron drag in wires separated by a soft electrostatic barrier of width ≳80 nm (ref. 12), which was interpreted as resulting entirely from momentum transfer. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by ~15 nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron sub-bands occupied in each wire, and interpret the results both in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different sub-band occupancies, the positive drag effect can be as large as 25%.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.182.html

+1: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.197.html

Spin Relaxation in InGaN Quantum Disks in GaN Nanowires

Animesh Banerjee†, Fatih Doğan‡, Junseok Heo†, Aurelien Manchon‡, Wei Guo†, and Pallab Bhattacharya*† DOI: 10.1021/nl203091f

The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of 100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott–Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D’yakonov–Perel’ relaxation mechanism in the presence of Rashba spin–orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.

http://pubs.acs.org/doi/abs/10.1021/nl203091f

Joule Heating and Spin-Transfer Torque Investigated on the Atomic Scale Using a Spin-Polarized Scanning Tunneling Microscope

S. Krause*, G. Herzog, A. Schlenhoff, A. Sonntag, and R. Wiesendanger DOI: 10.1103/PhysRevLett.107.186601

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.

http://link.aps.org/doi/10.1103/PhysRevLett.107.186601

Coulomb stability of the 4π-periodic Josephson effect of Majorana fermions

B. van Heck, F. Hassler, A. R. Akhmerov, and C. W. J. Beenakker DOI: 10.1103/PhysRevB.84.180502

The Josephson energy of two superconducting islands containing Majorana fermions is a 4π-periodic function of the superconducting phase difference. If the islands have a small capacitance, their ground state energy is governed by the competition of Josephson and charging energies. We calculate this ground-state energy in a ring geometry, as a function of the flux Φ enclosed by the ring, and show that the dependence on the Aharonov-Bohm phase 2eΦ/ℏ remains 4π periodic regardless of the ratio of charging and Josephson energies—provided that the entire ring is in a topologically nontrivial state. If part of the ring is topologically trivial, then the charging energy induces quantum phase slips that restore the usual 2π periodicity.

http://link.aps.org/doi/10.1103/PhysRevB.84.180502

Spin-orbit coupling induced enhancement of superconductivity in a two-dimensional repulsive gas of fermions

Oskar Vafek and Luyang Wang DOI: 10.1103/PhysRevB.84.172501

We study a model of a two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling αR and investigate the superconducting instability using the renormalization-group approach. We find that, in general, superconductivity is enhanced as the dimensionless ratio 1/2mαR2/EF increases, resulting in unconventional superconducting states which break time-reversal symmetry.

http://link.aps.org/doi/10.1103/PhysRevB.84.172501

Reading and writing charge on graphene devices

M. R. Connolly, E. D. Herbschleb, R. K. Puddy, M. Roy, D. Anderson, G. A. C. Jones, P. Maksym, C. G. Smith

We use a combination of charge writing and scanning gate microscopy to map and modify the local charge neutrality point of graphene field-effect devices. We give a demonstration of the technique by writing remote charge in a thin dielectric layer over the graphene-metal interface and detecting the resulting shift in local charge neutrality point. We perform electrostatic simulations to characterize the gating effect of a realistic scanning probe tip on a graphene bilayer and find a good agreement with the experimental results.

http://xxx.lanl.gov/abs/1111.0560

The influence of anisotropic gate potentials on the phonon induced spin-flip rate in GaAs quantum dots

Sanjay Prabhakar, Roderick V. N. Melnik, Luis L. Bonilla

We study the anisotropic orbital effect in the electric field tunability of the phonon induced spin-flip rate in quantum dots (QDs). Our study shows that anisotropic gate potential enhances the spin-flip rate and reduces the level crossing point to a lower quantum dot radius due to the suppression of the Land$\acute{e}$ g-factor towards bulk crystal. In the range of $10^4-10^6$ V/cm, the electric field tunability of the phonon induced spin-flip rate can be manipulated through strong Dresselhaus spin-orbit coupling. These results might assist the development of a spin based solid state quantum computer by manipulating phonon induced spin-flip rate through spin-orbit coupling with the application of anisotropic gate potential in a regime where the g-factor changes its sign.

http://xxx.lanl.gov/abs/1111.0558

Enhancement of shot noise due to the fluctuation of Coulomb interaction

Duo Li, Lei Zhang, Fuming Xu, Jian Wang

We have developed a theoretical formalism to investigate the contribution of fluctuation of Coulomb interaction to the shot noise based on Keldysh non-equilibrium Green's function method. We have applied our theory to study the behavior of dc shot noise of atomic junctions using the method of nonequilibrium Green's function combined with the density functional theory (NEGF-DFT). In particular, for atomic carbon wire consisting 4 carbon atoms in contact with two Al(100) electrodes, first principles calculation within NEGF-DFT formalism shows a negative differential resistance (NDR) region in I-V curve at finite bias due to the effective band bottom of the Al lead. We have calculated the shot noise spectrum using the conventional gauge invariant transport theory with Coulomb interaction considered explicitly on the Hartree level along with exchange and correlation effect. Although the Fano factor is enhanced from 0.6 to 0.8 in the NDR region, the expected super-Poissonian behavior in the NDR regionis not observed. When the fluctuation of Coulomb interaction is included in the shot noise, our numerical results show that the Fano factor is greater than one in the NDR region indicating a super-Poissonian behavior.

http://xxx.lanl.gov/abs/1111.0112

Datta-Das transistor in the quantum Hall regime

Luca Chirolli, D. Venturelli, F. Taddei, Rosario Fazio, V. Giovannetti

We propose a mechanism to couple spin-resolved edge states in the integer quantum Hall effect by employing an array of voltage-controlled top gates. Strong enhancement of the coupling is achieved when the array periodicity matches the inverse of the wave-vector difference of the two states involved. Well known techniques of separately contacting the edge states make possible to selectively populate and read-out the edge states, allowing full spin read-out. Our device represents the quantum Hall version of the all-electrical Datta-Das spin-field effect transistor.

http://xxx.lanl.gov/abs/1111.0675

Okt. 21. - Okt. 27. (2011)

Válogatta: Csonka Szabolcs

Charge detection in a bilayer graphene quantum dot Stefan Fringes, Christian Volk, Caroline Norda, Bernat Terrés, Jan Dauber, Stephan Engels, Stefan Trellenkamp, Christoph Stampfer

http://arxiv.org/abs/1110.5811

Tunable capacitive inter-dot coupling in a bilayer graphene double quantum dot

Stefan Fringes, Christian Volk, Bernat Terrés, Jan Dauber, Stephan Engels, Stefan Trellenkamp, Christoph Stampfer

http://arxiv.org/abs/1110.5803

Joule-assisted silicidation for short-channel silicon nanowire devices

Massimo Mongillo, Panayotis Spathis, Georgios Katsaros, Pascal Gentile, Marc Sanquer, Silvano De Franceschi

http://arxiv.org/abs/1110.5668

Majorana fermions in superconducting helical magnets

Ivar Martin, Alberto F. Morpurgo

http://arxiv.org/abs/1110.5637

Magnetoresistance of individual ferromagnetic GaAs/(Ga,Mn)As core-shell nanowires

Christian H. Butschkow, Elisabeth Reiger, Stefan Geißler, Andreas Rudolph, Marcello Soda, Dieter Schuh, Georg Woltersdorf, Werner Wegscheider, Dieter Weiss

http://arxiv.org/abs/1110.5507

Voltage induced conformational changes and current control in charge transfer through molecules

Lars Kecke, Joachim Ankerhold

http://arxiv.org/abs/1110.5505

Design of new superconducting materials, and point contact spectroscopy as a probe of strong electron correlations

Laura H. Greene, Hamood Z. Arham, Cassandra R. Hunt, Wan Kyu Park

http://arxiv.org/abs/1110.4742

Quantum confined electronic states in atomically well-defined graphene nanostructures

Sampsa Hämäläinen, Zhixiang Sun, Mark P. Boneschanscher, Andreas Uppstu, Mari Ijäs, Ari Harju, Daniël Vanmaekelbergh, Peter Liljeroth

http://arxiv.org/abs/1110.4208

How to distinguish specular from retro Andreev reflection in graphene rings

Jörg Schelter, Björn Trauzettel, Patrik Recher

http://arxiv.org/abs/1110.4383

Au40: A Large Tetrahedral Magic Cluster

De-en Jiang, Michael Walter

40 is a magic number for tetrahedral symmetry predicted in both nuclear physics and the electronic jellium model. We show that Au40 could be such a a magic cluster from density functional theory-based basin hopping for global minimization. The putative global minimum found for Au40 has a twisted pyramid structure, reminiscent of the famous tetrahedral Au20, and a sizable HOMO-LUMO gap of 0.69 eV, indicating its molecular nature. Analysis of the electronic states reveals that the gap is related to shell closings of the metallic electrons in a tetrahedrally distorted effective potential.

http://arxiv.org/abs/1110.4556

The Kondo effect in the presence of the Rashba spin-orbit interaction

Rok Zitko, Janez Bonca

We study the temperature scale of the Kondo screening of a magnetic impurity which hybridizes with a two-dimensional electron gas in the presence of the Rashba spin-orbit interaction. The problem is mapped to an effective single-band impurity model with a hybridization function having an inverse-square-root divergence at the bottom of the band. We study the effect of this divergence on the Kondo screening. The problem is solved numerically without further approximations using the numerical renormalization group technique. We find that the Rashba interaction leads to a small variation of the Kondo temperature (increase or decrease) which depends on the values of the impurity parameters.

http://arxiv.org/abs/1110.4566

Joule Heating and Spin-Transfer Torque Investigated on the Atomic Scale Using a Spin-Polarized Scanning Tunneling Microscope

S. Krause, G. Herzog, A. Schlenhoff, A. Sonntag, and R. Wiesendanger

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.

http://prl.aps.org/abstract/PRL/v107/i18/e186601

Gate-Dependent Orbital Magnetic Moments in Carbon Nanotubes

T. S. Jespersen, K. Grove-Rasmussen, K. Flensberg, J. Paaske, K. Muraki, T. Fujisawa, and J. Nygård

We investigate how the orbital magnetic moments of electron and hole states in a carbon nanotube quantum dot depend on the number of carriers on the dot. Low temperature transport measurements are carried out in a setup where the device can be rotated in an applied magnetic field, thus enabling accurate alignment with the nanotube axis. The field dependence of the level structure is measured by excited state spectroscopy and excellent correspondence with a single-particle calculation is found. In agreement with band structure calculations we find a decrease of the orbital magnetic moment with increasing electron or hole occupation of the dot, with a scale given by the band gap of the nanotube.

http://prl.aps.org/abstract/PRL/v107/i18/e186802

Magneto-Coulomb Effect in Carbon Nanotube Quantum Dots Filled with Magnetic Nanoparticles

S. Datta, L. Marty, J. P. Cleuziou, C. Tilmaciu, B. Soula, E. Flahaut, and W. Wernsdorfer

Electrical transport measurements of carbon nanotubes filled with magnetic iron nanoparticles are reported. Low-temperature (40 mK) magnetoresistance measurements showed conductance hysteresis with sharp jumps at the switching fields of the nanoparticles. Depending on the gate voltage, positive or negative hysteresis was observed. The results are explained in terms of a magneto-Coulomb effect: The spin flip of the iron island at a nonzero magnetic field causes a shift of the chemical potential induced by the change of Zeeman energy; i.e., an effective charge variation is detected by the nanotube quantum dot.

http://prl.aps.org/abstract/PRL/v107/i18/e186804

Enhanced NMR Relaxation of Tomonaga-Luttinger Liquids and the Magnitude of the Carbon Hyperfine Coupling in Single-Wall Carbon Nanotubes

A. Kiss, A. Pályi, Y. Ihara, P. Wzietek, P. Simon, H. Alloul, V. Zólyomi, J. Koltai, J. Kürti, B. Dóra, and F. Simon

Recent transport measurements [Churchill et al. Nature Phys. 5 321 (2009)] found a surprisingly large, 2–3 orders of magnitude larger than usual 13C hyperfine coupling (HFC) in 13C enriched single-wall carbon nanotubes. We formulate the theory of the nuclear relaxation time in the framework of the Tomonaga-Luttinger liquid theory to enable the determination of the HFC from recent data by Ihara et al. [ Europhys. Lett. 90 17 004 (2010)]. Though we find that 1/T1 is orders of magnitude enhanced with respect to a Fermi-liquid behavior, the HFC has its usual, small value. Then, we reexamine the theoretical description used to extract the HFC from transport experiments and show that similar features could be obtained with HFC-independent system parameters.

http://prl.aps.org/abstract/PRL/v107/i18/e187204

Okt. 13. - Okt. 20. (2011)

Válogatta: Pályi András

Field-induced polarization of Dirac valleys in bismuth

Zengwei Zhu, Aurélie Collaudin, Benoît Fauqué, Woun Kang, Kamran Behnia

The electronic structure of certain crystal lattices can contain multiple degenerate ’valleys’ for their charge carriers to occupy. This valley degree of freedom could be useful in the development of electronic devices. The principal challenge in the development of ’valleytronics’ is to lift the valley degeneracy of charge carriers in a controlled way. Here we show that in semi-metallic bismuth the flow of Dirac fermions along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. At high temperature and low magnetic field, bismuth’s three valleys are interchangeable and the three-fold symmetry of its lattice is maintained. As the temperature is decreased or the magnetic field increased, this symmetry is spontaneously lost. This loss may be an experimental manifestation of the recently proposed valley-nematic Fermi liquid state.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2111.html

Efficient quantum computing using coherent photon conversion

N. K. Langford, S. Ramelow, R. Prevedel, W. J. Munro, G. J. Milburn and A. Zeilinger

Single photons are excellent quantum information carriers: they were used in the earliest demonstrations of entanglement and in the production of the highest-quality entanglement reported so far. However, current schemes for preparing, processing and measuring them are inefficient. For example, down-conversion provides heralded, but randomly timed, single photons, and linear optics gates are inherently probabilistic. Here we introduce a deterministic process—coherent photon conversion (CPC)—that provides a new way to generate and process complex, multiquanta states for photonic quantum information applications. The technique uses classically pumped nonlinearities to induce coherent oscillations between orthogonal states of multiple quantum excitations. One example of CPC, based on a pumped four-wave-mixing interaction, is shown to yield a single, versatile process that provides a full set of photonic quantum processing tools. This set satisfies the DiVincenzo criteria for a scalable quantum computing architecture, including deterministic multiqubit entanglement gates (based on a novel form of photon–photon interaction), high-quality heralded single- and multiphoton states free from higher-order imperfections, and robust, high-efficiency detection. It can also be used to produce heralded multiphoton entanglement, create optically switchable quantum circuits and implement an improved form of down-conversion with reduced higher-order effects. Such tools are valuable building blocks for many quantum-enabled technologies. Finally, using photonic crystal fibres we experimentally demonstrate quantum correlations arising from a four-colour nonlinear process suitable for CPC and use these measurements to study the feasibility of reaching the deterministic regime with current technology. Our scheme, which is based on interacting bosonic fields, is not restricted to optical systems but could also be implemented in optomechanical, electromechanical and superconducting systems with extremely strong intrinsic nonlinearities. Furthermore, exploiting higher-order nonlinearities with multiple pump fields yields a mechanism for multiparty mediation of the complex, coherent dynamics.

http://www.nature.com/nature/journal/v478/n7369/full/nature10463.html

Electrical probe for mechanical vibrations in suspended carbon nanotubes

N. Traverso Ziani, G. Piovano, F. Cavaliere, and M. Sassetti

The transport properties of a suspended carbon nanotube probed by means of a scanning tunnel microscope (STM) tip are investigated. A microscopic theory of the coupling between electrons and mechanical vibrations is developed. It predicts a position-dependent coupling constant, sizable only in the region where the vibron is located. This fact has profound consequences on the transport properties, which allow to extract information on the location and size of the vibrating portions of the nanotube.

http://link.aps.org/doi/10.1103/PhysRevB.84.155423

Ultraefficient Cooling of Resonators: Beating Sideband Cooling with Quantum Control

Xiaoting Wang, Sai Vinjanampathy, Frederick W. Strauch, and Kurt Jacobs

The present state of the art in cooling mechanical resonators is a version of sideband cooling. Here we present a method that uses the same configuration as sideband cooling—coupling the resonator to be cooled to a second microwave (or optical) auxiliary resonator—but will cool significantly colder. This is achieved by varying the strength of the coupling between the two resonators over a time on the order of the period of the mechanical resonator. As part of our analysis, we also obtain a method for fast, high-fidelity quantum information transfer between resonators.

http://link.aps.org/doi/10.1103/PhysRevLett.107.177204

Coherent Control of Two Nuclear Spins Using the Anisotropic Hyperfine Interaction

Yingjie Zhang, Colm A. Ryan, Raymond Laflamme, and Jonathan Baugh

We demonstrate coherent control of two nuclear spins mediated by the magnetic resonance of a hyperfine-coupled electron spin. This control is used to create a double-nuclear coherence in one of the two electron spin manifolds, starting from an initial thermal state, in direct analogy to the creation of an entangled (Bell) state from an initially pure unentangled state. We identify challenges and potential solutions to obtaining experimental gate fidelities useful for quantum information processing in this type of system.

http://link.aps.org/doi/10.1103/PhysRevLett.107.170503

Experimentally Faking the Violation of Bell’s Inequalities

Ilja Gerhardt, Qin Liu, Antía Lamas-Linares, Johannes Skaar, Valerio Scarani, Vadim Makarov, andChristian Kurtsiefer

Entanglement witnesses such as Bell inequalities are frequently used to prove the nonclassicality of a light source and its suitability for further tasks. By demonstrating Bell inequality violations using classical light in common experimental arrangements, we highlight why strict locality and efficiency conditions are not optional, particularly in security-related scenarios.

http://link.aps.org/doi/10.1103/PhysRevLett.107.170404

Current-induced switching in transport through anisotropic magnetic molecules

Niels Bode, Liliana Arrachea, Gustavo Lozano, Tamara S. Nunner, Felix von Oppen

Anisotropic single-molecule magnets may be thought of as molecular switches, with possible applications to molecular spintronics. In this paper, we consider current-induced switching in single-molecule junctions containing an anisotropic magnetic molecule. We assume that the carriers interact with the magnetic molecule through the exchange interaction and focus on the regime in which the molecular spin dynamics is slow compared to the electronic tunneling rates. In this limit, the molecular spin obeys a non-equilibrium Langevin equation which takes the form of a generalized Landau-Lifshitz-Gilbert equation and which we derive microscopically by means of a non-equilibrium Born-Oppenheimer approximation. We exploit this Langevin equation to identify the relevant switching mechanisms and to derive the current-induced switching rates. As a byproduct, we also derive S-matrix expressions for the various torques entering into the Landau-Lifshitz-Gilbert equation which generalize previous expressions in the literature to non-equilibrium situations.

http://arxiv.org/abs/1110.4270

Effects of Interface Disorder on Valley Splitting in SiGe/Si/SiGe Quantum Wells

Zhengping Jiang, Neerav Kharche, Timothy Boykin, Gerhard Klimeck

A sharp potential barrier at the Si/SiGe interface introduces valley splitting (VS), which lifts the 2-fold valley degeneracy in strained SiGe/Si/SiGe quantum wells (QWs). This work examines in detail the effects of Si/SiGe interface disorder on the VS in an atomistic tight binding approach based on statistical sampling. VS is analyzed as a function of electric field, QW thickness, and simulation domain size. Strong electric fields push the electron wavefunctions into the SiGe buffer and introduce significant VS fluctuations from device to device. A Gedankenexperiment with ordered alloys sheds light on the importance of different bonding configurations on VS. We conclude that a single SiGe band offset and effective mass cannot comprehend the complex Si/SiGe interface interactions that dominate VS.

http://arxiv.org/abs/1110.4097

Okt. 07. - Okt. 13. (2011)

Válogatta: Tóvári Endre

Tunable metal-insulator transition in double-layer graphene heterostructures

L. A. Ponomarenko, A. A. Zhukov, R. Jalil, S. V. Morozov, K. S. Novoselov, V.V. Cheianov, V.I. Fal'ko, K. Watanabe, T. Taniguchi, A. K. Geim, R. V. Gorbachev

We report a double-layer electronic system made of two closely-spaced but electrically isolated graphene monolayers sandwiched in boron nitride. For large carrier densities in one of the layers, the adjacent layer no longer exhibits a minimum metallic conductivity at the neutrality point, and its resistivity diverges at low temperatures. This divergence can be suppressed by magnetic field or by reducing the carrier density in the adjacent layer. We believe that the observed localization is intrinsic for neutral graphene with generic disorder if metallic electron-hole puddles are screened out.

http://arxiv.org/abs/1107.0115

Published in Nature Physics (09 October 2011)

Mapping the Density of Scattering Centers Limiting the Electron Mean Free Path in Graphene

Filippo Giannazzo, Sushant Sonde, Raffaella Lo Nigro, Emanuele Rimini, and Vito Raineri

Recently, giant carrier mobility μ (>105 cm2 V–1 s–1) and micrometer electron mean free path (l) have been measured in suspended graphene or in graphene encapsulated between inert and ultraflat BN layers. Much lower μ values (10000–20000 cm2 V–1 s–1) are typically reported in graphene on common substrates (SiO2, SiC) used for device fabrication. The debate on the factors limiting graphene electron mean free path is still open with charged impurities (CI) and resonant scatterers (RS) indicated as the most probable candidates. As a matter of fact, the inhomogeneous distribution of such scattering sources in graphene is responsible of nanoscale lateral inhomogeneities in the electronic properties, which could affect the behavior of graphene nanodevices. Hence, high resolution two-dimensional (2D) mapping of their density is very important. Here, we used scanning capacitance microscopy/spectroscopy to obtain 2D maps of l in graphene on substrates with different dielectric permittivities, that is, SiO2 (κSiO2 = 3.9), 4H-SiC (0001) (κSiC = 9.7) and the very-high-κ perovskite strontium titanate, SrTiO3 (001), briefly STO (κSTO = 330). After measuring l versus the gate bias Vg on an array of points on graphene, maps of the CI density (NCI) have been determined by the neutrality point shift from Vg = 0 V in each curve, whereas maps of the RS density (NRS) have been extracted by fitting the dependence of l on the carrier density (n). Laterally inhomogeneous densities of CI and RS have been found. The RS distribution exhibits an average value 3 × 1010 cm–2 independently on the substrate. For the first time, a clear correlation between the minima in the l map and the maxima in the NCI map is obtained for graphene on SiO2 and 4H-SiC, indicating that CI are the main source of the lateral inhomogeneity of l. On the contrary, the l and NCI maps are uncorrelated in graphene on STO, while a clear correlation is found between l and NRS maps. This demonstrates a very efficient dielectric screening of CI in graphene on STO and the role of RS as limiting factor for electron mean free path.

http://pubs.acs.org/doi/abs/10.1021/nl2020922

Raman Signature of Graphene Superlattices

Victor Carozo, Clara M. Almeida, Erlon H. M. Ferreira, Luiz Gustavo Cançado, Carlos Alberto Achete, and Ado Jorio

When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An atomic force microscopy system is used to produce and identify specific rotationally stacked bilayer graphenes that demonstrate the validity of our model.

http://pubs.acs.org/doi/abs/10.1021/nl201370m

Optical Force Stamping Lithography

Spas Nedev, Alexander S. Urban, Andrey A. Lutich, and Jochen Feldmann

Here we introduce a new paradigm of far-field optical lithography, optical force stamping lithography. The approach employs optical forces exerted by a spatially modulated light field on colloidal nanoparticles to rapidly stamp large arbitrary patterns comprised of single nanoparticles onto a substrate with a single-nanoparticle positioning accuracy well beyond the diffraction limit. Because the process is all-optical, the stamping pattern can be changed almost instantly and there is no constraint on the type of nanoparticle or substrates used.

http://pubs.acs.org/doi/abs/10.1021/nl203214n

Anomalous Optoelectronic Properties of Chiral Carbon Nanorings...and One Ring to Rule Them All

Bryan M. Wong, Jonathan W. Lee

Carbon nanorings are hoop-shaped, {\pi}-conjugated macrocycles which form the fundamental annular segments of single-walled carbon nanotubes (SWNTs). In a very recent report, the structures of chiral carbon nanorings (which may serve as chemical templates for synthesizing chiral nanotubes) were experimentally synthesized and characterized for the first time. Here, in our communication, we show that the excited-state properties of these unique chiral nanorings exhibit anomalous and extremely interesting optoelectronic properties, with excitation energies growing larger as a function of size (in contradiction with typical quantum confinement effects). While the first electronic excitation in armchair nanorings is forbidden with a weak oscillator strength, we find that the same excitation in chiral nanorings is allowed due to a strong geometric symmetry breaking. Most importantly, among all the possible nanorings synthesized in this fashion, we show that only one ring, corresponding to a SWNT with chiral indices (n+3,n+1), is extremely special with large photoinduced transitions that are most readily observable in spectroscopic experiments.

http://xxx.lanl.gov/abs/1110.2756

Electron-Electron scattering and resistivity of ballistic multimode channels

K. E. Nagaev, N. Yu. Sergeeva

We show that electron--electron scattering gives a positive contribution to the resistivity of ballistic multimode wires whose width is much smaller than their length. This contribution is not exponentially small at low temperatures and therefore may be experimentally observable. It scales with temperature as $T^2$ for three-dimensional channels and as $T^{5/2}$ for two-dimensional ones.

http://xxx.lanl.gov/abs/1110.2607

Spin Manipulation and Relaxation in Spin-Orbit Qubits

Massoud Borhani, Xuedong Hu

We derive a generalized form of the Electric Dipole Spin Resonance (EDSR) Hamiltonian in the presence of the spin-orbit interaction for single spins in an elliptic quantum dot (QD) subject to an arbitrary (in both direction and magnitude) applied magnetic field. We predict a nonlinear behavior of the Rabi frequency as a function of the magnetic field for sufficiently large Zeeman energies, and present a microscopic expression for the anisotropic electron g-tensor. Similarly, an EDSR Hamiltonian is devised for two spins confined in a double quantum dot (DQD), where coherent Rabi oscillations between the singlet and triplet states are induced by jittering the inter-dot distance at the resonance frequency. Finally, we calculate two-electron-spin relaxation rates due to phonon emission, for both in-plane and perpendicular magnetic fields. Our results have immediate applications to current EDSR experiments on nanowire QDs, g-factor optimization of confined carriers, and spin decay measurements in DQD spin-orbit qubits.

http://xxx.lanl.gov/abs/1110.2193

Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes

Ferry Prins, Amelia Barreiro, Justus W. Ruitenberg, Johannes S. Seldenthuis, Nuria Aliaga-Alcalde, Lieven M. K. Vandersypen, Herre S. J. van der Zant

We report on a method to fabricate and measure gateable molecular junctions which are stable at room temperature. The devices are made by depositing molecules inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. The molecular junctions display gateable IV-characteristics at room temperature.

http://xxx.lanl.gov/abs/1110.2335

A spin quantum bit architecture with coupled donors and quantum dots in silicon

T. Schenkel, C. C. Lo, C. D. Weis, J. Bokor, A. M. Tyryshkin, S. A. Lyon

Spins of donor electrons and nuclei in silicon are promising quantum bit (qubit) candidates which combine long coherence times with the fabrication finesse of the silicon nanotechnology industry. We outline a potentially scalable spin qubit architecture where donor nuclear and electron spins are coupled to spins of electrons in quantum dots and discuss requirements for donor placement aligned to quantum dots by single ion implantation.

http://xxx.lanl.gov/abs/1110.2228

High-resolution spatial mapping of the temperature distribution of a Joule self-heated graphene nanoribbon

Young-Jun Yu, Melinda Y. Han, Stephane Berciaud, Alexandru B. Georgescu, Tony F. Heinz, Louis E. Brus, Kwang S. Kim, Philip Kim

We investigate the temperature distributions of Joule self-heated graphene nanoribbons (GNRs) with a spatial resolution finer than 100 nm by scanning thermal microscopy (SThM). The SThM probe is calibrated using the Raman G mode Stokes/anti-Stokes intensity ratio as a function of electric power applied to the GNR devices. From a spatial map of the temperature distribution, heat dissipation and transport pathways are investigated. By combining SThM and scanning gate microscopy data from a defected GNR, we observe hot spot formation at well-defined, localized sites.

http://xxx.lanl.gov/abs/1110.2984

Kinetics of spin relaxation in quantum wires and channels: Boundary spin echo and formation of a persistent spin helix

Valeriy A. Slipko and Yuriy V. Pershin

In this paper we use a spin kinetic equation to study spin-polarization dynamics in one-dimensional (1D) wires and 2D channels. The spin kinetic equation is valid in both diffusive and ballistic spin transport regimes and therefore is more general than the usual spin drift-diffusion equations. In particular, we demonstrate that in infinite 1D wires with Rashba spin-orbit interaction the exponential spin-relaxation decay can be modulated by an oscillating function. In the case of spin relaxation in finite length 1D wires, it is shown that an initially homogeneous spin polarization spontaneously transforms into a persistent spin helix. We find that a propagating spin-polarization profile reflects from a system boundary and returns back to its initial position similarly to the reflectance of sound waves from an obstacle. The Green’s function of the spin kinetic equation is derived for both finite and infinite 1D systems. Moreover, we demonstrate explicitly that the spin relaxation in specifically oriented 2D channels with Rashba and Dresselhaus spin-orbit interactions of equal strength occurs similarly to that in 1D wires of finite length. Finally, a simple transformation mapping 1D spin kinetic equation into the Klein-Gordon equation with an imaginary mass is found thus establishing an interesting connection between semiconductor spintronics and relativistic quantum mechanics.

http://prb.aps.org/abstract/PRB/v84/i15/e155306

Landau levels, edge states, and strained magnetic waveguides in graphene monolayers with enhanced spin-orbit interaction

Alessandro De Martino, Artur Hütten, and Reinhold Egger

The electronic properties of a graphene monolayer in a magnetic and a strain-induced pseudomagnetic field are studied in the presence of spin-orbit interactions (SOIs) that are artificially enhanced (e.g., by suitable adatom deposition). For the homogeneous case, we provide analytical results for the Landau level eigenstates for arbitrary intrinsic and Rashba SOIs, including also the Zeeman field. The edge states in a semi-infinite geometry are studied in the absence of the Rashba term. For a critical value of the magnetic field, we find a quantum phase transition separating two phases with spin-filtered helical edge states at the Dirac point. These phases have opposite spin current direction. We also discuss strained magnetic waveguides with inhomogeneous field profiles that allow for chiral snake orbits. Such waveguides are practically immune to disorder-induced backscattering, and the SOI provides nontrivial spin texture to these modes.

http://prb.aps.org/abstract/PRB/v84/i15/e155420

Szept. 30. - Okt. 06. (2011)

Válogatta: Balogh Zoltán

From Geneva to Italy Faster Than a Speeding Photon?

Adrian Cho

When news spread last week that physicists in Europe had spotted subatomic particles called neutrinos traveling faster than light, some of their colleagues reacted with incredulity. After all, the observation would contradict Einstein's special theory of relativity, which says that nothing can travel faster than light. Jim Al-Khalili, a theorist at the University of Surrey in the United Kingdom, even vowed to eat his boxer shorts on live television if the result holds up. But if it does, physicists won't be quite as bewildered as such reactions imply. Some have already developed a theoretical framework that can handle faster-than-light neutrinos and all other potential breaches of special relativity.

http://www.sciencemag.org/content/333/6051/1809.summary

Nobel Prize 2011: Perlmutter, Schmidt & Riess

Alison Wright

The 2011 Nobel Prize in Physics has been awarded to Saul Perlmutter, Brian Schmidt and Adam Riess, "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2131.html

Topology by dissipation in atomic quantum wires

Sebastian Diehl, Enrique Rico, Mikhail A. Baranov & Peter Zoller

Robust edge states and non-Abelian excitations are the trademark of topological states of matter, with promising applications such as ‘topologically protected’ quantum memory and computing. So far, topological phases have been exclusively discussed in a Hamiltonian context. Here we show that such phases and the associated topological protection and phenomena also emerge in open quantum systems with engineered dissipation. The specific system studied here is a quantum wire of spinless atomic fermions in an optical lattice coupled to a bath. The key feature of the dissipative dynamics described by a Lindblad master equation is the existence of Majorana edge modes, representing a non-local decoherence-free subspace. The isolation of the edge states is enforced by a dissipative gap in the p-wave paired bulk of the wire. We describe dissipative non-Abelian braiding operations within the Majorana subspace, and illustrate the insensitivity to imperfections. Topological protection is granted by a non-trivial winding number of the system density matrix.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2106.html

Ambipolar field effect in the ternary topological insulator (BixSb1–x)2Te3 by composition tuning

Desheng Kong, Yulin Chen, Judy J. Cha, Qianfan Zhang, James G. Analytis, Keji Lai, Zhongkai Liu,Seung Sae Hong, Kristie J. Koski, Sung-Kwan Mo, Zahid Hussain, Ian R. Fisher, Zhi-Xun Shen & Yi Cui

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties1, 2, 3, 4, 5, 6, 7, 8, 9, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping10, 11, 12, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping8, 9, 11 and electrical gating13, 14, 15, 16, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (BixSb1–x)2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (BixSb1–x)2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices17. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.172.html

Atomic Force Microscopy Based Tunable Local Anodic Oxidation of Graphene

Satoru Masubuchi, Miho Arai, and Tomoki Machida

We have fabricated graphene/graphene oxide/graphene (G/GO/G) junctions by local anodic oxidation lithography using atomic force microscopy (AFM). The conductance of the G/GO/G junction decreased with the bias voltage applied to the AFM cantilever Vtip. For G/GO/G junctions fabricated with large and small |Vtip|. GO was semi-insulating and semiconducting, respectively. AFM-based LAO lithography can be used to locally oxidize graphene with various oxidation levels and achieve tunability from semiconducting to semi-insulating GO.

http://pubs.acs.org/doi/abs/10.1021/nl201448q

MoS2 Nanoplates Consisting of Disordered Graphene-like Layers for High Rate Lithium Battery Anode Materials

Haesuk Hwang, Hyejung Kim, and Jaephil Cho

MoS2 nanoplates, consisting of disordered graphene-like layers, with a thickness of 30 nm were prepared by a simple, scalable, one-pot reaction using Mo(CO)6 and S in an autoclave. The product has a interlayer distance of 0.69 nm, which is much larger than its bulk counterpart (0.62 nm). This expanded interlater distance and disordered graphene-like morphology led to an excellent rate capability even at a 50C (53.1 A/g) rate, showing a reversible capacity of 700 mAh/g. In addition, a full cell (LiCoO2/MoS2) test result also demonstrates excellent capacity retention up to 60 cycles.

http://pubs.acs.org/doi/abs/10.1021/nl202675f

Mesoporous Manganese Oxide Nanowires for High-Capacity, High-Rate, Hybrid Electrical Energy Storage

Wenbo Yan, Talin Ayvazian, Jungyun Kim, Yu Liu, Keith C. Donavan, Wendong Xing, Yongan Yang, John C. Hemminger, and Reginald M. Penner

Arrays of mesoporous manganese dioxide, mp-MnO2, nanowires were electrodeposited on glass and silicon surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. The electrodeposition procedure involved the application, in a Mn(ClO4)2-containing aqueous electrolyte, of a sequence of 0.60 V (vs MSE) voltage pulses delineated by 25 s rest intervals. This “multipulse” deposition program produced mp-MnO2 nanowires with a total porosity of 43–56%. Transmission electron microscopy revealed the presence within these nanowires of a network of 3–5 nm diameter fibrils that were X-ray and electron amorphous, consistent with the measured porosity values. mp-MnO2 nanowires were rectangular in cross-section with adjustable height, ranging from 21 to 63 nm, and adjustable width ranging from 200 to 600 nm. Arrays of 20 nm × 400 nm mp-MnO2 nanowires were characterized by a specific capacitance, Csp, of 923 ± 24 F/g at 5 mV/s and 484 ± 15 F/g at 100 mV/s. These Csp values reflected true hybrid electrical energy storage with significant contributions from double-layer capacitance and noninsertion pseudocapacitance (38% for 20 nm × 400 nm nanowires at 5 mV/s) coupled with a Faradaic insertion capacity (62%). These two contributions to the total Csp were deconvoluted as a function of the potential scan rate.

http://pubs.acs.org/doi/abs/10.1021/nn2029583#cor1

Low Bias Electron Scattering in Structure-Identified Single Wall Carbon Nanotubes: Role of Substrate Polar Phonons

Bhupesh Chandra, Vasili Perebeinos, Stéphane Berciaud, Jyoti Katoch, Masa Ishigami, Philip Kim, Tony F. Heinz, and James Hone

We have performed temperature-dependent electrical transport measurements on known structure single wall carbon nanotubes at low bias. The experiments show a superlinear increase in nanotube resistivity with temperature, which is in contradiction with the linear dependence expected from nanotube acoustic-phonon scattering. The measured electron mean free path is also much lower than expected, especially at medium to high temperatures (>100 K). A theoretical model that includes scattering due to surface polar phonon modes of the substrates reproduces the experiments very well. The role of surface phonons is further confirmed by resistivity measurements of nanotubes on aluminum nitride.

http://prl.aps.org/abstract/PRL/v107/i14/e146601

Qubit state detection using the quantum Duffing oscillator

V. Leyton, M. Thorwart, and V. Peano

We introduce a detection scheme for the state of a qubit that is based on resonant few-photon transitions in a driven nonlinear resonator. The latter is parametrically coupled to the qubit and is used as its detector. Close to the fundamental resonator frequency, the nonlinear resonator shows sharp resonant few-photon transitions. Depending on the qubit state, these few-photon resonances are shifted to different driving frequencies. We show that this detection scheme offers the advantage of small back action, a large discrimination power with an enhanced readout fidelity, and a sufficiently large measurement efficiency. A realization of this scheme in the form of a persistent current qubit inductively coupled to a driven SQUID detector in its nonlinear regime is discussed.

http://prb.aps.org/abstract/PRB/v84/i13/e134501

Szept. 23. - Szept. 29. (2011)

Válogatta: Piszter Gábor

Stacking-dependent band gap and quantum transport in trilayer graphene

W. Bao, L. Jing, J. Velasco Jr, Y. Lee, G. Liu, D. Tran, B. Standley, M. Ayko, S. B. Cronin, D. Smirnov, M. Koshino, E. McCann, M. Bockrath and C. N. Lau

Graphene is an extraordinary two-dimensional (2D) system with chiral charge carriers and fascinating electronic, mechanical and thermal properties. In multilayer graphene, stacking order provides an important yet rarely explored degree of freedom for tuning its electronic properties. For instance, Bernal-stacked trilayer graphene (B-TLG) is semi-metallic with a tunable band overlap, and rhombohedral-stacked trilayer graphene (r-TLG) is predicted to be semiconducting with a tunable band gap. These multilayer graphenes are also expected to exhibit rich novel phenomena at low charge densities owing to enhanced electronic interactions and competing symmetries. Here we demonstrate the dramatically different transport properties in TLG with different stacking orders, and the unexpected spontaneous gap opening in charge neutral r-TLG. At the Dirac point, B-TLG remains metallic, whereas r-TLG becomes insulating with an intrinsic interaction-driven gap ~6 meV. In magnetic fields, well-developed quantum Hall (QH) plateaux in r-TLG split into three branches at higher fields. Such splitting is a signature of the Lifshitz transition, a topological change in the Fermi surface, that is found only in r-TLG. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2103.html

The experimental observation of quantum Hall effect of l = 3 chiral quasiparticles in trilayer graphene

Liyuan Zhang, Yan Zhang, Jorge Camacho, Maxim Khodas and Igor Zaliznyak

The linear dispersion of the low-energy electronic structure of monolayer graphene supports chiral quasiparticles that obey the relativistic Dirac equation and have a Berry phase of π. In bilayer graphene, the shape of the energy bands is quadratic, and its quasiparticles have a chiral degree, l = 2, and a Berry phase of 2π. These characteristics are usually determined from quantum Hall effect (QHE) measurements in which the Berry phase causes shifts in Shubnikov–de Haas (SdH) resistance oscillations. The QHE in graphene also exhibits an unconventional sequence of plateaux of Hall conductivity, SZIGMA_xy, with quantized steps of 4e^2=h, except for the first plateau, where it is governed by the Berry phase. Here, we report magnetotransport measurements in ABC-stacked trilayer graphene, and their variation with carrier density, magnetic field and temperature. Our results provide the first evidence of the presence of l = 3 chiral quasiparticles with cubic dispersion, predicted to occur in ABC-stacked trilayer graphene. The SdH oscillations we observe suggest Landau levels with four-fold degeneracy, a Berry phase of 3π, and the marked increase of cyclotron mass near charge neutrality. We also observe the predicted unconventional sequence of QHE plateaux, SZIGMA_xy = +- 6e^2/h,+-10e^2/h, and so on.

http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys2104.html

Observation of an electrically tunable band gap in trilayer graphene

Chun Hung Lui, Zhiqiang Li, Kin Fai Mak, Emmanuele Cappelluti and Tony F. Heinz

A striking feature of bilayer graphene is the induction of a significant band gap in the electronic states by the application of a perpendicular electric field. Thicker graphene layers are also highly attractive materials. The ability to produce a band gap in these systems is of great fundamental and practical interest. Both experimental and theoretical investigations of graphene trilayers with the typical ABA layer stacking have, however, revealed the lack of any appreciable induced gap. Here we contrast this behaviour with that exhibited by graphene trilayers with ABC crystallographic stacking. The symmetry of this structure is similar to that of AB-stacked graphene bilayers and, as shown by infrared conductivity measurements, permits a large band gap to be formed by an applied electric field. Our results demonstrate the critical and hitherto neglected role of the crystallographic stacking sequence on the induction of a band gap in few-layer graphene.

http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys2102.html

Exchange-Induced Electron Transport in Heavily Phosphorus-Doped Si Nanowires

Tae-Eon Park, Byoung-Chul Min, Ilsoo Kim, Jee-Eun Yang, Moon-Ho Jo, Joonyeon Chang, and Heon-Jin Choi

Heavily phosphorus-doped silicon nanowires (Si NWs) show intriguing transport phenomena at low temperature. As we decrease the temperature, the resistivity of the Si NWs initially decreases, like metals, and starts to increase logarithmically below a resistivity minimum temperature (Tmin), which is accompanied by (i) a zero-bias dip in the differential conductance and (ii) anisotropic negative magnetoresistance (MR), depending on the angle between the applied magnetic field and current flow. These results are associated with the impurity band conduction and electron scattering by the localized spins at phosphorus donor states. The analysis on the MR reveals that the localized spins are coupled antiferromagnetically at low temperature via the exchange interaction.

http://pubs.acs.org/doi/abs/10.1021/nl202535d

Direct Observation of Electron Confinement in Epitaxial Graphene Nanoislands

Soo-hyon Phark, Jérome Borme, Augusto León Vanegas, Marco Corbetta, Dirk Sander, and Jürgen Kirschner

One leading question for the application of graphene in nanoelectronics is how electronic properties depend on the size at the nanoscale. Direct observation of the quantized electronic states is central to conveying the relationship between electronic structures and local geometry. Scanning tunneling spectroscopy was used to measure differential conductance dI/dV patterns of nanometer-size graphene islands on an Ir(111) surface. Energy-resolved dI/dV maps clearly show a spatial modulation, indicating a modulated local density of states due to quantum confinement, which is unaffected by the edge configuration. We establish the energy dispersion relation with the quantized electron wave vector obtained from a Fourier analysis of dI/dV maps. The nanoislands preserve the Dirac Fermion properties with a reduced Fermi velocity.

http://pubs.acs.org/doi/abs/10.1021/nn2028105

Robust Au-Ag-Au Bimetallic Atom-Scale Junctions Fabricated by Self-Limited Ag Electrodeposition at Au Nanogaps

Tai-Wei Hwang and Paul W. Bohn

Atom-scale junctions (ASJs) exhibit quantum conductance behavior and have potential both for fundamental studies of adsorbate- ediated conductance in mesoscopic conductors and as chemical sensors. Electrochemically fabricated ASJs, in particular, show the stability needed for molecular detection applications. However, achieving physically robust ASJs at high yield is a challenge because it is difficult to control the direction and kinetics of metal deposition. In this work, a novel electrochemical approach is reported, in which Au-Ag-Au bimetallic ASJs are reproducibly fabricated from an initially prepared Au nanogap by sequential overgrowth and self-limited thinning. Applying a potential across specially prepared Au nanoelectrodes in the presence of aqueous Ag(I) leads to preferential galvanic reactions resulting in the deposition of Ag and the formation of an atom-scale junction between the electrodes. An external resistor is added in series with the ASJ to control self-termination, and adjusting solution chemical potential (concentration) is used to mediate self-thinning of junctions. The result is long-lived, mechanically stable ASJs that, unlike previous constructions, are stable in flowing solution, as well as to changes in solution media. These bimetallic ASJs exhibit a number of behaviors characteristic of quantum structures, including long-lived fractional conductance states, that are interpreted to arise from two or more quantized ASJs in series.

http://pubs.acs.org/doi/abs/10.1021/nn203404k

Reduced Graphene Oxide (rGO)- Wrapped Fullerene (C60) Wires

Jieun Yang, Mihee Heo, Hyo Joong Lee, Su-Moon Park, Jin Young Kim, and Hyeon Suk Shin

The assembly of reduced graphene oxide (rGO) and fullerene (C60) into hybrid (rGO/C60) wires was successfully performed by employing the liquid-liquid interfacial precipitation method. The rGO sheets spontaneously wrapped C60 wires through the π-π interaction between rGO and C60. Structural characterization of the rGO/C60 wires was carried out by using UV/visible spectroscopy, scanning electron microscopy, and transmission electron microscopy. FET devices with rGO/C60 wires were fabricated to investigate their electrical properties. The Ids-Vg curves of the hybrid wires exhibited p-type semiconducting behavior both in vacuum and in air, indicating hole transport through rGO as a shell layer, whereas pure C60 wires and rGO sheets showed n-type and ambipolar behaviors, respectively, under vacuum. Possible application of the fabricated wires, such as photovoltaic devices, was also demonstrated.

http://pubs.acs.org/doi/abs/10.1021/nn203073q

Interferometric and Noise Signatures of Majorana Fermion Edge States in Transport Experiments

Grégory Strübi, Wolfgang Belzig, Mahn-Soo Choi, and C. Bruder

Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown–Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers.

http://prl.aps.org/abstract/PRL/v107/i13/e136403

Phase Diffusion in Graphene-Based Josephson Junctions

I.V. Borzenets, U. C. Coskun, S. J. Jones, and G. Finkelstein

We report on graphene-based Josephson junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to ~2 K, at which point we can detect a small, but nonzero, resistance. We attribute this resistance to the phase diffusion mechanism, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further characterize the nature of the electromagnetic environment and dissipation in our samples.

http://prl.aps.org/abstract/PRL/v107/i13/e137005

Finite-Bias Cooper Pair Splitting

L. Hofstetter, Sz. Csonka, A. Baumgartner, G. Fülöp, S. d’Hollosy, J. Nygard, and C. Schönenberger

In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit nonclassical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the nonlocal electrical transport through the device can be tuned by electrical means and that the energy dependence of the effective density of states in the QDs is relevant for the rates of Cooper pair splitting (CPS) and elastic cotunneling. Such experimental tools are necessary to understand and develop CPS-based sources of entangled electrons in solid-state devices.

http://prl.aps.org/abstract/PRL/v107/i13/e136801

Szept. 15. - Szept. 22. (2011)

Válogatta: Fülöp Gergő

On-demand single-electron transfer between distant quantum dots

R. P. G. McNeil, M. Kataoka, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, I. Farrer & D. A. Ritchie

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times—a cumulative distance of 0.25 mm—without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

http://www.nature.com/nature/journal/v477/n7365/full/nature10444.html

pdf

Strong back-action of a linear circuit on a single electronic quantum channel

F. D. Parmentier, A. Anthore, S. Jezouin, H. le Sueur, U. Gennser, A. Cavanna, D. Mailly & F. Pierre

The question of which laws govern electricity in mesoscopic circuitsis a fundamental matter that also has direct implications for the quantum engineering of nanoelectronic devices. When a quantum-coherent conductor is inserted into a circuit, its transport properties are modified; in particular, its conductance is reduced because of the circuit back-action. This phenomenon, known as environmental Coulomb blockade, results from the granularity of charge transfers across the coherent conductor1. Although extensively studied for a tunnel junction in a linear circuit2, 3, 4, 5, it is only fully understood for arbitrary short coherent conductors in the limit of small circuit impedances and small conductance reduction6, 7, 8. Here, we investigate experimentally the strong-back-action regime, with a conductance reduction of up to 90%. This is achieved by embedding a single quantum channel of tunable transmission in an adjustable on-chip circuit of impedance comparable to the resistance quantum RK = h/e2 at microwave frequencies. The experiment reveals significant deviations from calculations performed in the weak back-action framework6, 7, and is in agreement with recent theoretical results9, 10. Based on these measurements, we propose a generalized expression for the conductance of an arbitrary quantum channel embedded in a linear circuit.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2092.html

pdf

Spin Polarization Measurement of Homogeneously Doped Fe_1–xCo_xSi Nanowires by Andreev Reflection Spectroscopy

John P. DeGrave†, Andrew L. Schmitt†, Rachel S. Selinsky†, Jeremy M. Higgins†, David J. Keavney‡, and Song Jin*†

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe1–xCoxSi alloy nanowires with = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.

http://pubs.acs.org/doi/abs/10.1021/nl2026426

Thermoelectricity in Fullerene–Metal Heterojunctions

Shannon K. Yee†, Jonathan A. Malen†, Arun Majumdar*‡, and Rachel A. Segalman*§

[...] Herein, we report molecular junction thermoelectric measurements of fullerene molecules (i.e., C60, PCBM, and C70) trapped between metallic electrodes (i.e., Pt, Au, Ag). Fullerene junctions demonstrate the first strongly n-type molecular thermopower corresponding to transport through the LUMO, and the highest measured magnitude of molecular thermopower to date. While the electronic conductance of fullerenes is highly variable, due to fullerene’s variable bonding geometries with the electrodes, the thermopower shows predictable trends based on the alignment of the LUMO with the work function of the electrodes. Both the magnitude and trend of the thermopower suggest that heterostructuring organic and inorganic materials at the nanoscale can further enhance thermoelectric performance, therein providing a new pathway for designing thermoelectric materials.

http://pubs.acs.org/doi/abs/10.1021/nl2014839

Mobility-Dependent Low-Frequency Noise in Graphene Field-Effect Transistors '

Yan Zhang†, Emilio E. Mendez†‡, and Xu Du†

We have investigated the low-frequency 1/f noise of both suspended and on-substrate graphene field-effect transistors and its dependence on gate voltage, in the temperature range between 300 and 30 K. We have found that the noise amplitude away from the Dirac point can be described by a generalized Hooge’s relation in which the Hooge parameter αH is not constant but decreases monotonically with the device’s mobility, with a universal dependence that is sample and temperature independent. The value of αH is also affected by the dynamics of disorder, which is not reflected in the DC transport characteristics and varies with sample and temperature. We attribute the diverse behavior of gate voltage dependence of the noise amplitude to the relative contributions from various scattering mechanisms, and to potential fluctuations near the Dirac point caused by charge carrier inhomogeneity. The higher carrier mobility of suspended graphene devices accounts for values of 1/f noise significantly lower than those observed in on-substrate graphene devices and most traditional electronic materials.

http://pubs.acs.org/doi/abs/10.1021/nn202749z

Impurity effects on Fabry-Perot physics of ballistic carbon nanotubes

F. Romeo, R. Citro, A. Di Bartolomeo

We present a theoretical model accounting for the anomalous Fabry-Perot pattern observed in the ballistic conductance of a single-wall carbon nanotubes. Using the scattering field theory, it is shown that the presence of a limited number of impurities along the nanotube can be identified by a measurement of the conductance and their position determined. Impurities can be made active or silent depending on the interaction with the substrate via the back-gate. The conceptual steps for designing a bio-molecules detector are briefly discussed.

http://lanl.arxiv.org/abs/1109.1104v1

Signature of Majorana Fermions in Charge Transport in Semiconductor Nanowires

Chunlei Qu, Yongping Zhang, Li Mao, Chuanwei Zhang

We investigate the charge transport in a semiconductor nanowire that is subject to a perpendicular magnetic field and in partial contact with an s-wave superconductor. We find that Majorana fermions, existing at the interface between superconducting and normal sections of the nanowire within certain parameter region, can induce resonant Andreev reflection of electrons at the interface, which yields a zero energy peak in the electrical conductance of the nanowire. The width of the zero energy conductance peak for different experimental parameters is characterized. While the zero energy peak provides a signature for Majorana fermions in one dimensional nanowires, it disappears in a two-dimensional semiconductor thin film with the same experimental setup because of the existence of other edge states in two dimensions. The proposed charge transport experiment may provide a simple and experimentally feasible method for the detection of Majorana fermions in semiconductor nanowires.

http://xxx.lanl.gov/abs/1109.4108

Integer Quantum Hall Effect in Trilayer Graphene '

A. Kumar1, W. Escoffier1, J. M. Poumirol1, C. Faugeras1, D. P. Arovas2, M. M. Fogler2, F. Guinea3, S. Roche4,5, M. Goiran1, and B. Raquet1

By using high-magnetic fields (up to 60 T), we observe compelling evidence of the integer quantum Hall effect in trilayer graphene. The magnetotransport fingerprints are similar to those of the graphene monolayer, except for the absence of a plateau at a filling factor of ν=2. At a very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. The measured Hall resistivity plateaus are well reproduced theoretically, using a self-consistent Hartree calculations of the Landau levels and assuming an ABC stacking order of the three layers.

http://prl.aps.org/abstract/PRL/v107/i12/e126806

Shubnikov-de Haas oscillations of a single layer graphene under dc current bias

Zhenbing Tan, Changling Tan, Li Ma, G. T. Liu, L. Lu, and C. L. Yang*

Shubnikov-de Haas (SdH) oscillations under a dc current bias are experimentally studied on a Hall bar sample of single-layer graphene. In dc resistance, the bias current shows the common damping effect on the SdH oscillations and the effect can be well accounted for by an elevated electron temperature that is found to be linearly dependent on the current bias. In differential resistance, a novel phase inversion of the SdH oscillations has been observed with increasing dc bias, namely we observe the oscillation maxima develop into minima and vice versa. Moreover, it is found that the onset bias current, at which a SdH extremum is about to invert, is linearly dependent on the magnetic field of the SdH extrema. These observations are quantitatively explained with the help of a general SdH formula.

http://prb.aps.org/abstract/PRB/v84/i11/e115429

Az alábbiak régiek.

High Current Density Esaki Tunnel Diodes Based on GaSb-InAsSb Heterostructure Nanowires

http://pubs.acs.org/doi/abs/10.1021/nl202180b

Electronic Double Slit Interferometers Based on Carbon Nanotubes

http://pubs.acs.org/doi/abs/10.1021/nl202360h

Observation of Raman G-Peak Split for Graphene Nanoribbons with Hydrogen-Terminated Zigzag Edges

http://pubs.acs.org/doi/abs/10.1021/nl201387x

Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators

http://pubs.acs.org/doi/abs/10.1021/nl201275q

Synthesis of Graphene Nanoribbons Encapsulated in Single-Walled Carbon Nanotubes

http://pubs.acs.org/doi/abs/10.1021/nl2024678

Szept. 08. - Szept. 14. (2011)

Válogatta: Scherübl Zoltán

In situ tunable g factor for a single electron confined inside an InAs quantum dot

W. Liu1, S. Sanwlani1, R. Hazbun2, J. Kolodzey2, A. S. Bracker3, D. Gammon3, and M. F. Doty1

Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the g-factor for a single confined electron can be tuned in situ by over 50% with a minimal change in applied voltage.

http://prb.aps.org/abstract/PRB/v84/i12/e121304

Hole-spin initialization and relaxation times in InAs/GaAs quantum dots

F. Fras, B. Eble, P. Desfonds, F. Bernardot, C. Testelin, and M. Chamarro

We study, at low temperature and zero magnetic field, the hole-spin dynamics in InAs/GaAs quantum dots. We measure the hole-spin relaxation time at a time scale longer than the dephasing time (about ten nanoseconds), imposed by the hole-nuclear hyperfine coupling. We use a pump-probe configuration and compare two experimental techniques based on differential absorption. The first one works in the time domain, and the second one is a new experimental method, the dark-bright time-scanning spectroscopy (DTS), working in the frequency domain. The measured hole-spin relaxation times, using these two techniques, are very similar, in the order of TNh≈1 μs. It is mainly imposed by the inhomogeneous hole hyperfine coupling in the hole localization volume. The DTS technique allows us also to measure the hole-spin initialization time τi. The hole spin is initialized by a periodic train of circularly polarized pulses at 76 MHz; we have observed that τi decreases as the power density increases, and we have measured a minimum value of τi≈100 ns in good agreement with a simple model [see B. Eble, P. Desfonds, F. Fras, F. Bernardot, C. Testelin, M. Chamarro, A. Miard and A. Lemaître Phys. Rev. B 81 045322 (2010)].

http://prb.aps.org/abstract/PRB/v84/i12/e125431

Efficient terahertz emission from InAs nanowires

Denis V. Seletskiy1,4,*, Michael P. Hasselbeck1, Jeffrey G. Cederberg2, Aaron Katzenmeyer3, Maria E. Toimil-Molares3, François Léonard3, A. Alec Talin3,†, and Mansoor Sheik-Bahae1

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is ∼15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

http://prb.aps.org/abstract/PRB/v84/i11/e115421

Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators

Ivana Vobornik*†, Unnikrishnan Manju‡, Jun Fujii†, Francesco Borgatti§, Piero Torelli†, Damjan Krizmancic†, Yew San Hor, Robert J. Cava, and Giancarlo Panaccione†

We complete our recently introduced theoretical framework treating the double-quantum-dot system with a generalized form of Hubbard model. The effects of all quantum parameters involved in our model on the charge-stability diagram are discussed in detail. A general formulation of the microscopic theory is presented and, truncating at one orbital per site, we study the implication of different choices of the model confinement potential on the Hubbard parameters as well as the charge-stability diagram. We calculate the charge-stability diagram keeping three orbitals per site and find that the effect of additional higher-lying orbitals on the subspace with lowest-energy orbitals only can be regarded as a small renormalization of Hubbard parameters, thereby justifying our practice of keeping only the lowest orbital in all other calculations. The role of the harmonic-oscillator frequency in the implementation of the Gaussian model potential is discussed, and the effect of an external magnetic field is identified to be similar to choosing a more localized electron wave function in microscopic calculations. The full matrix form of the Hamiltonian, including all possible exchange terms and several peculiar charge-stability diagrams due to unphysical parameters, is presented in the Appendices, thus emphasizing the critical importance of a reliable microscopic model in obtaining the system parameters defining the Hamiltonian.

http://prb.aps.org/abstract/PRB/v84/i11/e115301

Direct measurement of quantum phases in graphene via photoemission spectroscopy

Choongyu Hwang1, Cheol-Hwan Park2, David A. Siegel1,2, Alexei V. Fedorov3, Steven G. Louie1,2,*, and Alessandra Lanzara1,2,

Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berry's phase and the sign of the matrix element of the Hamiltonian between two nonequivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g. , dynamic phases and spin or valley degeneracy, and the absence of methodology. Here, we report a way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for nonequivalent orbitals. Upon rotating light polarization by π/2, we found that graphene with a Berry's phase of nπ (n=1 for single- and n=2 for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by π/n, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a technique to directly extract fundamental quantum electronic information on a variety of materials.

http://prb.aps.org/abstract/PRB/v84/i12/e125422

Current correlations in the interacting Cooper-pair beam-splitter

J. Rech, D. Chevallier, T. Jonckheere, T. Martin

Using a conserving many-body treatment, we propose an approach allowing the computation of currents and their correlations in interacting multi-terminal mesoscopic systems involving quantum dots coupled to normal and/or superconducting leads. We illustrate our method with the Cooper-pair beam-splitter setup recently proposed, which we model as a double quantum dot with weak interactions, connected to a superconducting lead and two normal ones. Our results suggest that even a weak Coulomb repulsion tends to favor positive current cross-correlations.

http://xxx.lanl.gov/abs/1109.2476

Quantum Hall effect and semimetallic behavior in dual-gated ABA trilayer graphene

E. A. Henriksen, D. Nandi, J. P. Eisenstein

The electronic structure of multilayer graphenes depends strongly on the number of layers as well as the stacking order. Here we explore the electronic transport of purely ABA-stacked trilayer graphenes in a dual-gated field effect device configuration. We find both the quantum Hall effect (QHE) and low-field transport to be distinctly different from the mono- and bilayer graphenes, showing electron-hole asymmetries that are strongly suggestive of a semimetallic band overlap. When subject to an electric field perpendicular to the sheet, Landau level splittings due to breaking of the lattice mirror symmetry are clearly observed.

http://xxx.lanl.gov/abs/1109.2385

Measuring the complex admittance of a carbon nanotube double quantum dot

S.J. Chorley, J. Wabnig, Z.V. Penfold-Fitch, K.D. Petersson, J. Frake, C.G. Smith, M.R. Buitelaar

We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. Besides being of fundamental interest, our results present an important step forward towards non-invasive charge and spin state readout in carbon nanotube quantum dots.

http://xxx.lanl.gov/abs/1109.1827

Imaging the lateral shift of a quantum-point contact using scanning-gate microscopy

S. Schnez, C. Rössler, T. Ihn, K. Ensslin, C. Reichl, W. Wegscheider

We perform scanning-gate microscopy on a quantum-point contact. It is defined in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs heterostructure, giving rise to a weak disorder potential. The lever arm of the scanning tip is significantly smaller than that of the split gates defining the conducting channel of the quantum-point contact. We are able to observe that the conducting channel is shifted in real space when asymmetric gate voltages are applied. The observed shifts are consistent with transport data and numerical estimations.

http://xxx.lanl.gov/abs/1109.1544

Carbon tips as electrodes for single-molecule junctions

Andres Castellanos-Gomez, Stefan Bilan, Linda A. Zotti, Carlos R. Arroyo, Nicolas Agrait, Juan Carlos Cuevas, Gabino Rubio-Bollinger

We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.

http://xxx.lanl.gov/abs/1109.2089

Szept. 01. - Szept. 07. (2011)

Válogatta: Pósa László

Cím

Authors: ???

Abstract

http://???

@@ Line 35: / Line 35: @@
 -----
+'''Electronic and plasmonic phenomena at graphene grain boundaries
+''Graphene1, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications12. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (~10–20 nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.
+ Z. Fei,
+A. S. Rodin,
+W. Gannett,
+S. Dai,
+W. Regan,
+M. Wagner,
+M. K. Liu,
+A. S. McLeod,
+G. Dominguez,
+M. Thiemens,
+Antonio H. Castro Neto,
+F. Keilmann,
+A. Zettl,
+R. Hillenbrand,
+M. M. Fogler
+& D. N. Basov
+[http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.197.html]
 == Szept. 30. - Okt. 5. (2013)==

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