Recent interesting articles

When a low-energy electron is incident on an interface between a metal and superconductor, it causes the injection of a Cooper pair into the superconductor and the generation of a hole that reflects back into the metal—a process known as Andreev reflection. In confined geometries, this process can give rise to discrete Andreev bound states (ABS), which can enable transport of supercurrents through non-superconducting materials and have recently been proposed as a means of realizing solid-state qubits1, 2, 3. Here, we report transport measurements of sharp, gate-tunable ABS formed in a superconductor–quantum dot (QD)–normal system realized on an exfoliated graphene sheet. The QD is formed in graphene beneath a superconducting contact as a result of a work-function mismatch4, 5. Individual ABS form when the discrete QD levels are proximity-coupled to the superconducting contact. Owing to the low density of states of graphene and the sensitivity of the QD levels to an applied gate voltage, the ABS spectra are narrow and can be continuously tuned down to zero energy by the gate voltage.

http://dx.doi.org/10.1038/nphys1911

Single-layer MoS_2 transistors

B. Radisavljevic,A. Radenovic, J. Brivio, V. Giacometti and A. Kis

Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene1, 2, both because of its rich physics3, 4, 5 and its high mobility6. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors7. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films8, 9, 10, 11, 12, 13 or requires high voltages14, 15. Although single layers of MoS2 have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5–3 cm2 V−1 s−1 range17 are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS2 mobility of at least 200 cm2 V−1 s−1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 108 and ultralow standby power dissipation. Because monolayer MoS2 has a direct bandgap16, 18, it can be used to construct interband tunnel FETs19, which offer lower power consumption than classical transistors. Monolayer MoS2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

http://dx.doi.org/10.1038/nnano.2010.279

The origins and limits of metal–graphene junction resistance

Fengnian Xia, Vasili Perebeinos, Yu-ming Lin, Yanqing Wu and Phaedon Avouris

A high-quality junction between graphene and metallic contacts is crucial in the creation of high-performance graphene transistors. In an ideal metal–graphene junction, the contact resistance is determined solely by the number of conduction modes in graphene. However, as yet, measurements of contact resistance have been inconsistent, and the factors that determine the contact resistance remain unclear. Here, we report that the contact resistance in a palladium–graphene junction exhibits an anomalous temperature dependence, dropping significantly as temperature decreases to a value of just 110 ± 20 Ω µm at 6 K, which is two to three times the minimum achievable resistance. Using a combination of experiment and theory we show that this behaviour results from carrier transport in graphene under the palladium contact. At low temperature, the carrier mean free path exceeds the palladium–graphene coupling length, leading to nearly ballistic transport with a transfer efficiency of ~75%. As the temperature increases, this carrier transport becomes less ballistic, resulting in a considerable reduction in efficiency.

http://dx.doi.org/10.1038/nnano.2011.6

Does a ferromagnet with spin-dependent masses produce a spin-filtering effect in a ferromagnetic/insulator/superconductor junction?

Gaetano Annunziata, Mario Cuoco, Paola Gentile, Alfonso Romano, Canio Noce

We analyze charge transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. We take into account the possibility that ferromagnetism in the first electrode may be driven by a mass renormalization of oppositely polarized carriers, i.e. by a spin bandwidth asymmetry, rather than by a rigid splitting of up-and down-spin electron bands as in a standard Stoner ferromagnet. By evaluating the averaged charge conductance for both an s- and a $d_{x^2-y^2}$-wave order parameter for the S side, we show that the mass mismatch in the ferromagnetic electrode may mimic a spin active barrier. Indeed, in the $s$-wave case we show that under suitable conditions the spin dependent conductance of minority carriers below the energy gap $\Delta_0$ can be larger than for majority carriers, and lower above $\Delta_0$. On the other hand, for a d_{x^2-y^2}-wave superconductor similar spin-dependent effects give rise to an asymmetric peak splitting in the conductance. These results suggest that the junction may work as a spin-filtering device.

http://arxiv.org/abs/1101.4780v1

Novel E-beam lithography technique for in-situ junction fabrication: the controlled undercut

Florent Lecocq (NEEL), Cécile Naud (NEEL), Ioan M. Pop (NEEL), Zhi-Hui Peng (NEEL), Iulian Matei (NEEL), Thierry Crozes (NEEL), Thierry Fournier (NEEL), Wiebke Guichard (NEEL), Olivier Buisson (NEEL)

We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by E-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in-situ fabrication of junctions and capacitors without the use of the well-known suspended bridge[1]. The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 0.01 to more than 10000 micron square. We have fabricated Al/AlOx/Al Josephson junctions, phase qubit and capacitors using a 100kV E- beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20kV E-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillations decay and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this controlled undercut technique.

http://arxiv.org/abs/1101.4576v2

Tuning of the spin-orbit interaction in a quantum dot by an in-plane magnetic field

M.P. Nowak, B. Szafran, F.M. Peeters, B. Partoens, W. Pasek

Using an exact diagonalization approach we show that one- and two-electron InAs quantum dots exhibit avoided crossing in the energy spectra that are induced by the spin-orbit coupling in the presence of an in-plane external magnetic field. The width of the avoided crossings depends strongly on the orientation of the magnetic field which reveals the intrinsic anisotropy of the spin-orbit coupling interactions. We find that for specific orientations of the magnetic field avoided crossings vanish. Value of this orientation can be used to extract the ratio of the strength of Rashba and Dresselhaus interactions. The spin-orbit anisotropy effects for various geometries and orientations of the confinement potential are discussed. Our analysis explains the physics behind the recent measurements performed on a gated self-assembled quantum dot [S. Takahashi et al. Phys. Rev. Lett. 104, 246801 (2010)].

http://arxiv.org/abs/1102.1002v1

Self heating and nonlinear current-voltage characteristics in bilayer graphene

J. K. Viljas, A. Fay, M. Wiesner, P. J. Hakonen

We demonstrate by experiments and numerical simulations that the low-temperature current-voltage characteristics in diffusive bilayer graphene (BLG) exhibit a strong superlinearity at finite bias voltages. The superlinearity is weakly dependent on doping and on the length of the graphene sample. This effect can be understood as a result of Joule heating. It is stronger in BLG than in monolayer graphene (MLG), since the conductivity of BLG is more sensitive to temperature due to the higher density of electronic states at the Dirac point.

http://arxiv.org/abs/1102.0658v1

Vibrational Instabilities in Resonant Electron Transport through Single-Molecule Junctions

R. Härtle, M. Thoss

We analyze various limits of vibrationally coupled resonant electron transport in single-molecule junctions. Based on a master equation approach, we discuss analytic and numerical results for junctions under a high bias voltage or weak electronic-vibrational coupling. It is shown that in these limits the vibrational excitation of the molecular bridge increases indefinitely, i.e. the junction exhibits a vibrational instability. Moreover, our analysis provides analytic results for the vibrational distribution function and reveals that these vibrational instabilities are related to electron-hole pair creation processes.

http://arxiv.org/abs/1102.0840v1

Graphene quantum dots formed by a spatial modulation of the Dirac gap

G. Giavaras, Franco Nori

An electrostatic quantum dot cannot be formed in monolayer graphene, because of the Klein tunnelling. However, a dot can be formed with the help of a uniform magnetic field. As shown here, a spatial modulation of the Dirac gap leads to confined states with discrete energy levels, thus defining a dot, without applying external electric and magnetic fields. Gap-induced dot states can coexist and couple with states introduced by an electrostatic potential. This property allows the region in which the resulting states are localized to be tuned with the potential.

http://arxiv.org/abs/1102.1042v1

Realtime magnetic field sensing and imaging using a single spin in diamond

Rolf Simon Schoenfeld, Wolfgang Harneit

The Zeeman splitting of a localized single spin can be used to construct a magnetometer allowing high precision measurements of magnetic fields with almost atomic spatial resolution. While sub-{\mu}T sensitivity can in principle be obtained using pulsed techniques and long measurement times, a fast and easy-to-use method without laborious data post-processing is desirable for a scanning-probe approach with high spatial resolution. In order to measure the resonance frequency in realtime, we applied a field-frequency lock to the continuous wave ODMR signal of a single electron spin in a nanodiamond. In our experiment, we achieved a sampling rate of up to 100 readings per second with a sensitivity of 6 {\mu}T/$\sqrt{Hz}$. Using this method we have imaged the microscopic field distribution around a magnetic wire. Images with \sim 30 {\mu}T resolution and 4096 sub-micron sized pixels were acquired in 10 minutes. By measuring the field response of multiple spins on the same object we were able to partly reconstruct the orientation of the field.

http://arxiv.org/abs/1009.4138v2

Mechanical Deformation of Nanoscale Metal Rods: When Size and Shape Matter

M. J. Lagos, F. Sato, D. S. Galvão, and D. Ugarte

Face centered cubic metals deform mainly by propagating partial dislocations generating planar fault ribbons. How do metals deform if the size is smaller than the fault ribbons? We studied the elongation of Au and Pt nanorods by in situ electron microscopy and ab initio calculations. Planar fault activation barriers are so low that, for each temperature, a minimal rod size is required to become active for releasing elastic energy. Surface effects dominate deformation energetics; system size and shape determine the preferred fault gliding directions which induce different tensile and compressive behavior.

http://link.aps.org/doi/10.1103/PhysRevLett.106.055501

Breakdown of Atomic-Sized Metallic Contacts Measured on Nanosecond Scale

Shaoyin Guo, Joshua Hihath, and Nongjian Tao

We report on a study of atomic-sized metallic contacts on a time scale of nanoseconds using a combined DC and AC circuit. The approach leads to a time resolution 3−4 orders of magnitude faster than the measurements carried out to date, making it possible to observe fast transient conductance-switching events associated with the breakdown, re-formation, and atomic scale structural rearrangements of the contact. The study bridges the wide gap in the time scales between the molecular dynamic simulations and real world experiments, and the method may be applied to study nano- and subnanosecond processes in other nanoscale devices, such as molecular junctions.

http://pubs.acs.org/doi/abs/10.1021/nl1026748

Grain Boundary Mapping in Polycrystalline Graphene

Kwanpyo Kim†‡, Zonghoon Lee§, William Regan†‡, C. Kisielowski§, M. F. Crommie†‡, and A. Zettl†‡*

We report direct mapping of the grains and grain boundaries (GBs) of large-area monolayer polycrystalline graphene sheets, at large (several micrometer) and single-atom length scales. Global grain and GB mapping is performed using electron diffraction in scanning transmission electron microscopy (STEM) or using dark-field imaging in conventional TEM. Additionally, we employ aberration-corrected TEM to extract direct images of the local atomic arrangements of graphene GBs, which reveal the alternating pentagon−heptagon structure along high-angle GBs. Our findings provide a readily adaptable tool for graphene GB studies.

http://pubs.acs.org/doi/abs/10.1021/nn1033423

Universal Segregation Growth Approach to Wafer-Size Graphene from Non-Noble Metals

Nan Liu†, Lei Fu†, Boya Dai, Kai Yan, Xun Liu, Ruiqi Zhao, Yanfeng Zhang, and Zhongfan Liu*

Graphene has been attracting wide interests owing to its excellent electronic, thermal, and mechanical performances. Despite the availability of several production techniques, it is still a great challenge to achieve wafer-size graphene with acceptable uniformity and low cost, which would determine the future of graphene electronics. Here we report a universal segregation growth technique for batch production of high-quality wafer-scale graphene from non-noble metal films. Without any extraneous carbon sources, 4 in. graphene wafers have been obtained from Ni, Co, Cu−Ni alloy, and so forth via thermal annealing with over 82% being 1−3 layers and excellent reproducibility. We demonstrate the first example of monolayer and bilayer graphene wafers using Cu−Ni alloy by combining the distinct segregation behaviors of Cu and Ni. Together with the easy detachment from growth substrates, we believe this facile segregation technique will offer a great driving force for graphene research.

http://pubs.acs.org/doi/abs/10.1021/nl103962a#cor1

Graphene Spintronics: The Role of Ferromagnetic Electrodes

Jesse Maassen*†, Wei Ji*†‡, and Hong Guo†

We report a first principles study of spin transport under finite bias through a graphene−ferromagnet (FM) interface, where FM = Co(111), Ni(111). The use of Co and Ni electrodes achieves spin efficiencies reaching 80% and 60%, respectively. This large spin filtering results from the materials specific interaction between graphene and the FM which destroys the linear dispersion relation of the graphene bands and leads to an opening of spin-dependent energy gaps of ≈0.4−0.5 eV at the K points. The minority spin band gap resides higher in energy than the majority spin band gap located near EF, a feature that results in large minority spin dominated currents.

http://pubs.acs.org/doi/abs/10.1021/nl1031919

Thermal Dynamics of Graphene Edges Investigated by Polarized Raman Spectroscopy

Ya Nan Xu†‡, Da Zhan†, Lei Liu†, Hui Suo‡, Zhen Hua Ni§*, Thuong Thuong Nguyen, Chun Zhao‡, and Ze Xiang Shen†*

In this report, we present Raman spectroscopy investigation of the thermal stability and dynamics of graphene edges. It is found that graphene edges (both armchair and zigzag) are not stable and undergo modifications even at temperature as low as 200 °C. On the basis of polarized Raman results, we provide possible structural models on how graphene edges change during annealing. The zigzag edges rearrange and form armchair segments that are ±30° relative to the edge direction, while armchair edges are dominated by armchair segments even at annealing temperature as high as 500 °C. The modifications of edge structures by thermal annealing (zigzag segments rearrange in form of armchair segments) provide a flexible way to control the electronic properties of graphene and graphene nanostructures.

http://pubs.acs.org/doi/abs/10.1021/nn101920c

Fully Electrical Read-Write Device Out of a Ferromagnetic Semiconductor

S. Mark, P. Dürrenfeld, K. Pappert, L. Ebel, K. Brunner, C. Gould, and L. W. Molenkamp

We report the realization of a read-write device out of the ferromagnetic semiconductor (Ga,Mn)As as the first step to a fundamentally new information processing paradigm. Writing the magnetic state is achieved by current-induced switching and readout of the state is done by the means of the tunneling anisotropic magnetoresistance effect. This 1 bit demonstrator device can be used to design an electrically programmable memory and logic device.

http://prl.aps.org/abstract/PRL/v106/i5/e057204

Deterministic Entanglement of Photons in Two Superconducting Microwave Resonators

H. Wang1,2, Matteo Mariantoni1, Radoslaw C. Bialczak1, M. Lenander1, Erik Lucero1, M. Neeley1, A. D. O’Connell1, D. Sank1, M. Weides1, J. Wenner1, T. Yamamoto1,3, Y. Yin1, J. Zhao1, John M. Martinis1, and A. N. Cleland1,*

Quantum entanglement, one of the defining features of quantum mechanics, has been demonstrated in a variety of nonlinear spinlike systems. Quantum entanglement in linear systems has proven significantly more challenging, as the intrinsic energy level degeneracy associated with linearity makes quantum control more difficult. Here we demonstrate the quantum entanglement of photon states in two independent linear microwave resonators, creating N-photon NOON states (entangled states |N0⟩+|0N⟩) as a benchmark demonstration. We use a superconducting quantum circuit that includes Josephson qubits to control and measure the two resonators, and we completely characterize the entangled states with bipartite Wigner tomography. These results demonstrate a significant advance in the quantum control of linear resonators in superconducting circuits.

http://prl.aps.org/abstract/PRL/v106/i6/e060401

Jan. 09. - Jan. 26. (2011)

Válogatta: Makk Péter

Local charge of the ν = 5/2 fractional quantum Hall state

Vivek Venkatachalam, Amir Yacoby, loren Pfeiffer & Ken West

Electrons moving in two dimensions under the influence of strong magnetic fields effectively lose their kinetic energy and display exotic behaviour dominated by Coulomb forces. When the ratio of electrons to magnetic flux quanta in the system (ν) is near 5/2, the electrons are predicted to condense into a correlated phase with fractionally charged quasiparticles and a ground-state degeneracy that grows exponentially as these quasiparticles are introduced1. The only way for electrons to transform between the many ground states would be to braid the fractional excitations around each other. This property has been proposed as the basis of a fault-tolerant quantum computer2. Here we present observations of localized quasiparticles at ν = 5/2, confined to puddles by disorder. Using a local electrometer to compare how quasiparticles at ν = 5/2 and ν = 7/3 charge these puddles, we were able to extract the ratio of local charges for these states. Averaged over several disorder configurations and samples, we found the ratio to be 4/3, suggesting that the local charges are = e/3 and = e/4, where e is the charge of an electron. This is in agreement with theoretical predictions for a paired state at ν = 5/2. Confirming the existence of localized e/4 quasiparticles shows that proposed interferometry experiments to test statistics and computational ability of the state at ν = 5/2 would be possible.

http://www.nature.com/nature/journal/v469/n7329/full/nature09680.html

Novel E-beam lithography technique for in-situ junction fabrication: the controlled undercut

Florent Lecocq (NEEL), Cécile Naud (NEEL), Ioan M. Pop (NEEL), Zhi-Hui Peng (NEEL), Iulian Matei (NEEL), Thierry Crozes (NEEL), Thierry Fournier (NEEL), Wiebke Guichard (NEEL), Olivier Buisson (NEEL)

We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by E-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in-situ fabrication of junctions and capacitors without the use of the well-known suspended bridge[1]. The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 0.01 to more than 10000 micron square. We have fabricated Al/AlOx/Al Josephson junctions, phase qubit and capacitors using a 100kV E- beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20kV E-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillations decay and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this controlled undercut technique.

http://arxiv.org/abs/1101.4576

Quasi one-dimensional transport in single GaAs/AlGaAs core-shell nanowires

Damien Lucot, Fauzia Jabeen, Jean-Christophe Harmand, Gilles Patriarche, Romain Giraud, Giancarlo Faini, Dominique Mailly

We present an original approach to fabricate single GaAs/AlGaAs core-shell nanowire with robust and reproducible transport properties. The core-shell structure is buried in an insulating GaAs overlayer and connected as grown in a two probe set-up using the highly doped growth substrate and a top diffused contact. The measured conductance shows a non-ohmic behavior with temperature and voltage-bias dependences following power laws, as expected for a quasi-1D system.

http://arxiv.org/abs/1101.0421

Transition Voltage Spectroscopy and the Nature of Vacuum Tunneling

M.L. Trouwborst, C.A. Martin, R.H.M. Smit, C.M. Guedon, T.A. Baart, S.J. van der Molen, J.M. van Ruitenbeek

Transition Voltage Spectroscopy (TVS) has been proposed as a tool to analyze charge transport through molecular junctions. We extend TVS to Au-vacuum-Au junctions and study the distance dependence of the transition voltage V_t(d) for clean electrodes in cryogenic vacuum. On the one hand, this allows us to provide an important reference for V_t(d)-measurements on molecular junctions. On the other hand, we show that TVS forms a simple and powerful test for vacuum tunneling models.

http://arxiv.org/abs/1101.1779 http://pubs.acs.org/doi/abs/10.1021/nl103699t

A spin-filter device based on armchair graphene nanoribbons

A. Saffarzadeh, R. Farghadan

The coherent spin-polarized electron transport through a zigzag-edge graphene flake (ZGF), sandwiched between two semi-infinite armchair graphene nanoribbons, is investigated by means of Landauer-Buttiker formalism. To study the edge magnetism of the ZGF, we use the half-filled Hubbard model within the Hartree-Fock approximation. The results show that the junction acts as a spin filter with high degree of spin polarization in the absence of magnetic electrodes and external fields. By applying a gate voltage the spin-filtering efficiency of this device can be effectively controlled and the spin polarization can reach values as high as 90%.

http://arxiv.org/abs/1101.2948

Surprising lack of magnetism in the conductance channels of Pt atomic chains

Manohar Kumar, Oren Tal, Roel H.M. Smit, Jan M. van Ruitenbeek

Pt is known to show spontaneous formation of chains of metal atoms upon breaking a metallic contact. From model calculations these have been predicted to be spin polarized, which is reasonable in view of the Stoner enhanced susceptibility of bulk Pt and the increased density of states due to the reduced dimensionality. Here, we demonstrate that shot noise reveals information on the magnetic state of Pt atomic chains. Against all predictions, we find clear evidence for a non-magnetic ground state for the conductance channels of Pt atomic chains.

http://arxiv.org/abs/1101.3939

Cotunneling through a magnetic single-molecule transistor based on N\atC60

Nicolas Roch, Romain Vincent, Florian Elste, Wolfgang Harneit, Wolfgang Wernsdorfer, Carsten Timm, Franck Balestro

We present an experimental and theoretical study of a magnetic single-molecule transistor based on N@C60 connected to gold electrodes. Particular attention is paid to the regime of intermediate molecule-lead coupling, where cotunneling effects manifest themselves in the Coulomb-blockade regime. The experimental results for the differential conductance as a function of bias, gate voltage, and external magnetic field are in agreement with our analysis of the tunneling rates and provide evidence of magnetic signatures in single-N@C60 devices arising from an antiferromagnetic exchange interaction between the C60 spin and the nitrogen spin.

http://arxiv.org/abs/1101.4198

Negative differential resistance in scanning tunneling microscopy: simulations on C$_{60}$-based molecular overlayers

Frederico D. Novaes, Manuel Cobian, Alberto Garcia, Pablo Ordejon, Hiromu Ueba, Nicolas Lorente

We determine the conditions in which negative differential resistance (NDR) appears in the C$_{60}$-based molecular device of [Phys. Rev. Lett. {\bf 100}, 036807 (2008)] by means of ab-initio electron-transport simulations. Our calculations grant access to bias-dependent intrinsic properties of the molecular device, such as electronic levels and their partial widths. We show that these quantities depend on the molecule-molecule and molecule-electrode interactions of the device. Hence, NDR can be tuned by modifying the bias behavior of levels and widths using both types of interactions.

http://arxiv.org/abs/1101.3714

Electrical Detection of Surface Plasmon Polaritons by 1G0 Gold Quantum Point Contacts

Naomi Ittah and Yoram Selzer*

Electrical detection of surface plasmons polaritons (SPPs) is essential for realization of integrated fast nanoscale plasmonic circuits. We demonstrate electrical detection of SPPs by measuring their remote gating effect on 1G0 metal quantum point contacts (MQPC) made of gold. Gating is argued to take place by a photoassisted transport mechanism with nonmonotonic behavior of its magnitude as a function of distance between the MQPCs and the position of SPPs creation.

http://pubs.acs.org/doi/abs/10.1021/nl103398z

Observation of Hole Accumulation in Ge/Si Core/Shell Nanowires Using off-Axis Electron Holography

Luying Li*†, David J. Smith†, Eric Dailey‡, Prashanth Madras‡, Jeff Drucker†, and Martha R. McCartney† Hole accumulation in Ge/Si core/shell nanowires (NWs) has been observed and quantified using off-axis electron holography and other electron microscopy techniques. The epitaxial 110-oriented Ge/Si core/shell NWs were grown on Si (111) substrates by chemical vapor deposition through the vapor−liquid−solid growth mechanism. High-angle annular-dark-field scanning transmission electron microscopy images and off-axis electron holograms were obtained from specific NWs. The excess phase shifts measured by electron holography across the NWs indicated the presence of holes inside the Ge cores. Calculations based on a simplified coaxial cylindrical model gave hole densities of (0.4 ± 0.2) /nm3 in the core regions.

http://pubs.acs.org/doi/abs/10.1021/nl1033107

Toward Intrinsic Graphene Surfaces: A Systematic Study on Thermal Annealing and Wet-Chemical Treatment of SiO2-Supported Graphene Devices

Zengguang Cheng, Qiaoyu Zhou, Chenxuan Wang, Qiang Li, Chen Wang*, and Ying Fang*

By combining atomic force microscopy and trans-port measurements, we systematically investigated effects of thermal annealing on surface morphologies and electrical properties of single-layer graphene devices fabricated by electron beam lithography on silicon oxide (SiO2) substrates. Thermal treatment above 300 °C in vacuum was required to effectively remove resist residues on graphene surfaces. However, annealing at high temperature was found to concomitantly bring graphene in close contact with SiO2 substrates and induce increased coupling between them, which leads to heavy hole doping and severe degradation of mobilities in graphene devices. To address this problem, a wet-chemical approach employing chloroform was developed in our study, which was shown to enable both intrinsic surfaces and enhanced electrical properties of graphene devices. Upon the recovery of intrinsic surfaces of graphene, the adsorption and assisted fibrillation of amyloid β-peptide (Aβ1-42) on graphene were electrically measured in real time.

http://pubs.acs.org/doi/abs/10.1021/nl103977d

Scanning tunnelling microscopy: Closing in on molecular junctions

Andreas Heinrich

Contacts between a single molecule and a metal electrode can be good or bad depending on the number of metal atoms that are in direct contact with the molecule.

http://www.nature.com/nnano/journal/v6/n1/full/nnano.2011.266.html

Vibrational and electronic heating in nanoscale junction

Daniel R. Ward, David A. Corley, James M. Tour & Douglas Natelson

Understanding and controlling the flow of heat is a major challenge in nanoelectronics. When a junction is driven out of equilibrium by light or the flow of electric charge, the vibrational and electronic degrees of freedom are, in general, no longer described by a single temperature1, 2, 3, 4, 5, 6. Moreover, characterizing the steady-state vibrational and electronic distributions in situ is extremely challenging. Here, we show that surface-enhanced Raman emission may be used to determine the effective temperatures for both the vibrational modes and the electrons in the current in a biased metallic nanoscale junction decorated with molecules7. Molecular vibrations show mode-specific pumping by both optical excitation8 and d.c. current9, with effective temperatures exceeding several hundred kelvin. Anti-Stokes electronic Raman emission10, 11 indicates that the effective electronic temperature at bias voltages of a few hundred millivolts can reach values up to three times the values measured when there is no current. The precise effective temperatures are model-dependent, but the trends as a function of bias conditions are robust, and allow direct comparisons with theories of nanoscale heating.

http://www.nature.com/nnano/journal/v6/n1/full/nnano.2010.240.html

Gate-dependent spin–orbit coupling in multielectron carbon nanotubes

T. S. Jespersen, K. Grove-Rasmussen, J. Paaske, K. Muraki, T. Fujisawa, J. Nygård & K. Flensberg

Understanding how the orbital motion of electrons is coupled to the spin degree of freedom in nanoscale systems is central for applications in spin-based electronics and quantum computation. Here we demonstrate such spin–orbit coupling in a carbon-nanotube quantum dot in the general multielectron regime and in the presence of finite disorder. Also, we find a systematic dependence of the spin–orbit coupling on the electron occupation of the quantum dot. Such a dependence has not been seen in any other system and follows from the curvature-induced spin–orbit-split Dirac spectrum of the underlying graphene lattice. Our findings suggest that the spin–orbit coupling is a general property of carbon-nanotube quantum dots, which should provide a unique platform for the study of spin–orbit effects and their applications.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1880.html

Long-range Kondo signature of a single magnetic impurity

Henning Prüser, Martin Wenderoth, Piet E. Dargel, Alexander Weismann, Robert Peters, Thomas Pruschke & Rainer G. Ulbrich

The Kondo effect, one of the first recognized correlation phenomena in condensed-matter physics1, has regained attention because of scanning tunnelling spectroscopy (STS) experiments carried out on single magnetic impurities2, 3. Despite the subnanometre resolution capability of local probe techniques, one of the fundamental aspects of Kondo physics, its spatial extension, is still subject to discussion. Until now all STS studies on single adsorbed atoms have shown that observable Kondo features vanish rapidly with increasing distance from the impurity4, 5, 6, 7, 8, 9. Here we report on a hitherto unobserved long-range Kondo signature for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We present a theoretical interpretation of the measured signatures using a combined approach of band-structure and many-body numerical renormalization group calculations. These are in excellent agreement with the rich spatially and spectroscopically resolved experimental data.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1876.html

Atomic-scale engineering of electrodes for single-molecule contacts

Guillaume Schull, Thomas Frederiksen, Andrés Arnau, Daniel Sánchez-Portal & Richard Berndt

The transport of charge through a conducting material depends on the intrinsic ability of the material to conduct current and on the charge injection efficiency at the contacts between the conductor and the electrodes carrying current to and from the material1, 2, 3. According to theoretical considerations4, this concept remains valid down to the limit of single-molecule junctions5. Exploring this limit in experiments requires atomic-scale control of the junction geometry. Here we present a method for probing the current through a single C60 molecule while changing, one by one, the number of atoms in the electrode that are in contact with the molecule. We show quantitatively that the contact geometry has a strong influence on the conductance. We also find a crossover from a regime in which the conductance is limited by charge injection at the contact to a regime in which the conductance is limited by scattering at the molecule. Thus, the concepts of ‘good’ and ‘bad’ contacts, commonly used in macro- and mesoscopic physics, can also be applied at the molecular scale.

http://www.nature.com/nnano/journal/v6/n1/abs/nnano.2010.215.html

Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect

Luqiao Liu, Takahiro Moriyama, D. C. Ralph, and R. A. Buhrman

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance dynamics. The Oersted field from the current also generates a ferromagnetic resonance signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle.

http://prl.aps.org/abstract/PRL/v106/i3/e036601

Interference and Switching of Josephson Current Carried by Nonlocal Spin-Entangled Electrons in a SQUID-Like System with Quantum Dots

Zhi Wang and Xiao Hu

The Josephson current of spin-entangled electrons through the two branches of a SQUID-like structure with two quantum dots exhibits a magnetic-flux response different from the conventional Josephson current. Because of their interference, the period of maximum Josephson current changes from h/2e to h/e, which can be used for detecting the Cooper-pair splitting efficiency. The nonlocal spin entanglement provides a quantum mechanical functionale for switching on and off this novel Josephson current, and explicitly a switch is formulated by including a pilot junction. It is shown that the device can be used to measure the magnitude of split-tunneling Josephson current.

http://prl.aps.org/abstract/PRL/v106/i3/e037002

Dec. 16. - Jan. 09. (2011)

Válogatta: Csontos Miklós

Signatures of Wigner localization in epitaxially grown nanowires

L. H. Kristinsdóttir, J. C. Cremon, H. A. Nilsson, H. Q. Xu, L. Samuelson, H. Linke, A. Wacker, and S. M. Reimann

It was predicted by Wigner in 1934 that an electron gas will undergo a transition to a crystallized state when its density is very low. Whereas significant progress has been made toward the detection of electronic Wigner states, their clear and direct experimental verification still remains a challenge. Here we address signatures of Wigner molecule formation in the transport properties of InSb nanowire quantum-dot systems, where a few electrons may form localized states depending on the size of the dot (i.e., the electron density). Using a configuration interaction approach combined with an appropriate transport formalism, we are able to predict the transport properties of these systems, in excellent agreement with experimental data. We identify specific signatures of Wigner state formation, such as the strong suppression of the antiferromagnetic coupling, and are able to detect the onset of Wigner localization, both experimentally and theoretically, by studying different dot sizes.

http://prb.aps.org/abstract/PRB/v83/i4/e041101

Kondo Quantum Criticality of Magnetic Adatoms in Graphene

Bruno Uchoa, T. G. Rappoport, and A. H. Castro Neto

We examine the exchange Hamiltonian for magnetic adatoms in graphene with localized inner shell states. On symmetry grounds, we predict the existence of a class of orbitals that lead to a distinct class of quantum critical points in graphene, where the Kondo temperature scales as TK∝|J-Jc|1/3 near the critical coupling Jc, and the local spin is effectively screened by a super-Ohmic bath. For this class, the RKKY interaction decays spatially with a fast power law ∼1/R7. Away from half filling, we show that the exchange coupling in graphene can be controlled across the quantum critical region by gating. We propose that the vicinity of the Kondo quantum critical point can be directly accessed with scanning tunneling probes and gating.

http://prl.aps.org/abstract/PRL/v106/i1/e016801

Coexistence of Coulomb Blockade and Zero Bias Anomaly in a Strongly Coupled Nanodot

L. Bitton, D. B. Gutman, R. Berkovits, and A. Frydman

The current-voltage characteristics through a metallic nanoparticle which is well coupled to a metallic lead are measured. It is shown that the I–V curves are composed of two contributions. One is a suppression of the tunneling conductivity at the Fermi level, and the second is an oscillating feature which shifts with gate voltage. The results indicate that zero-bias anomaly and Coulomb blockade phenomena coexist in an asymmetric strongly coupled zero-dimensional system.

http://prl.aps.org/abstract/PRL/v106/i1/e016803

Válogatta: Makk Péter

Sublattice asymmetry and spin-orbit interaction induced out-of-plane spin polarization of photoelectrons

Peter Rakyta, Andor Kormanyos, Jozsef Cserti

We study theoretically the effect of spin-orbit coupling and sublattice asymmetry in graphene on the spin polarization of photoelectrons. We show that sublattice asymmetry in graphene not only opens a gap in the band structure but in case of finite spin-orbit interaction it also gives rise to an out-of-plane spin polarization of electrons close to the Dirac point of the Brillouin zone. This can be detected by measuring the spin polarization of photoelectrons and therefore spin resolved photoemission spectroscopy can reveal the presence of a band gap even if it is too small to be observed directly by angle resolved photoemission spectroscopy because of the finite resolution of measurements or because the sample is $p$-doped. We present analytical and numerical calculations on the energy and linewidth dependence of photoelectron intensity distribution and spin polarization.

http://arxiv.org/abs/1101.0624

Quasi one-dimensional transport in single GaAs/AlGaAs core-shell nanowires

Damien Lucot, Fauzia Jabeen, Jean-Christophe Harmand, Gilles Patriarche, Romain Giraud, Giancarlo Faini, Dominique Mailly

We present an original approach to fabricate single GaAs/AlGaAs core-shell nanowire with robust and reproducible transport properties. The core-shell structure is buried in an insulating GaAs overlayer and connected as grown in a two probe set-up using the highly doped growth substrate and a top diffused contact. The measured conductance shows a non-ohmic behavior with temperature and voltage-bias dependences following power laws, as expected for a quasi-1D system.

http://arxiv.org/abs/1101.0421

Real-time observation of discrete Andreev tunneling events

V. F. Maisi, O.-P. Saira, Yu. A. Pashkin, J. S. Tsai, D. V. Averin, J. P. Pekola

Electronic transport across a boundary between conductors with dissimilar carriers is a non-trivial process. Of particular interest in this respect is the transport through a superconductor - normal metal interface that at low energies is dominated by the so-called Andreev reflection. In this process, a Cooper pair in a superconductor is converted into two electrons (quasiparticles) in the normal metal, or, oppositely, two electrons combine into a pair in a reverse event. We have performed a real-time detection of these events and directly distinguished them from one-electron processes providing a direct proof of the existence of the two-electron Andreev transitions and allowing us to measure their rate. Since the rate depends on coherence properties of the wavefunctions of two electrons, it gives a fingerprint of the junction. The tunneling rates we observe indicate that the standard barriers are quite far from being atomically flat insulator layers.

http://arxiv.org/abs/1012.5750

Dec. 9. - Dec. 15. (2010)

Válogatta: Makk Péter

Spin reconstruction in quantum wires subject to a perpendicular magnetic field

Gilad Barak, Loren N. Pfeiffer, Ken W. West, Bertrand I. Halperin, Amir Yacoby

We study the effects of a perpendicular magnetic field on the spin and charge distribution across a quantum wire. Using momentum resolved tunneling between two parallel wires we measure the dispersion relation for different perpendicular magnetic fields. We find that as the magnetic field increases, a strip of spin polarized electrons separates in the cross section of the wire. We provide a quantitative description of the emerging structure within a Hartree-Fock approximation, showing that it results from exchange interaction between electrons in the wire. We discuss the relation of these results to the structure of Quantum Hall edge states.

http://arxiv.org/abs/1012.1845

Nonequilibrium transport in molecular junctions with strong electron-phonon interactions

R. C. Monreal, F. Flores, A. Martin-Rodero

We present a combined theoretical approach to study the nonequilibrium transport properties of nanoscale systems coupled to metallic electrodes and exhibiting strong electron-phonon interactions. We use the Keldysh Green function formalism to generalize beyond linear theory in the applied voltage an equation of motion method and an interpolative self-energy approximation previously developed in equilibrium. We analyze the specific characteristics of inelastic transport appearing in the intensity versus voltage curves and in the conductance, providing qualitative criteria for the sign of the step-like features in the conductance. Excellent overall agreement between both approaches is found for a wide range of parameters.

http://arxiv.org/abs/1012.2015

Probing Charge Transfer at Surfaces Using Graphene Transistors

Pierre L. Levesque*†‡, Shadi S. Sabri†§, Carla M. Aguirre†, Jonathan Guillemette†§, Mohamed Siaj†, Patrick Desjardins†, Thomas Szkopek*†§, and Richard Martel*†‡

Graphene field effect transistors (FETs) are extremely sensitive to gas exposure. Charge transfer doping of graphene FETs by atmospheric gas is ubiquitous but not yet understood. We have used graphene FETs to probe minute changes in electrochemical potential during high-purity gas exposure experiments. Our study shows quantitatively that electrochemistry involving adsorbed water, graphene, and the substrate is responsible for doping. We not only identify the water/oxygen redox couple as the underlying mechanism but also capture the kinetics of this reaction. The graphene FET is highlighted here as an extremely sensitive potentiometer for probing electrochemical reactions at interfaces, arising from the unique density of states of graphene. This work establishes a fundamental basis on which new electrochemical nanoprobes and gas sensors can be developed with graphene.

http://pubs.acs.org/doi/abs/10.1021/nl103015w

Nonvolatile Memory Functionality of ZnO Nanowire Transistors Controlled by Mobile Protons

Jongwon Yoon†§, Woong-Ki Hong†§, Minseok Jo†, Gunho Jo†, Minhyeok Choe†, Woojin Park†, Jung Inn Sohn‡, Stanko Nedic‡, Hyungsang Hwang†, Mark E. Welland‡, and Takhee Lee†*

We demonstrated the nonvolatile memory functionality of ZnO nanowire field effect transistors (FETs) using mobile protons that are generated by high-pressure hydrogen annealing (HPHA) at relatively low temperature (400 °C). These ZnO nanowire devices exhibited reproducible hysteresis, reversible switching, and nonvolatile memory behaviors in comparison with those of the conventional FET devices. We show that the memory characteristics are attributed to the movement of protons between the Si/SiO2 interface and the SiO2/ZnO nanowire interface by the applied gate electric field. The memory mechanism is explained in terms of the tuning of interface properties, such as effective electric field, surface charge density, and surface barrier potential due to the movement of protons in the SiO2 layer, consistent with the UV photoresponse characteristics of nanowire memory devices. Our study will further provide a useful route of creating memory functionality and incorporating proton-based storage elements onto a modified CMOS platform for FET memory devices using nanomaterials.

http://pubs.acs.org/doi/abs/10.1021/nn102633z

Dephasing time of GaAs electron-spin qubits coupled to a nuclear bath exceeding 200 μs

Hendrik Bluhm, Sandra Foletti, Izhar Neder, Mark Rudner, Diana Mahalu, Vladimir Umansky & Amir Yacoby

Qubits, the quantum mechanical bits required for quantum computing, must retain their quantum states for times long enough to allow the information contained in them to be processed. In many types of electron-spin qubits, the primary source of information loss is decoherence due to the interaction with nuclear spins of the host lattice. For electrons in gate-defined GaAs quantum dots, spin-echo measurements have revealed coherence times of about 1 μs at magnetic fields below 100 mT (refs 1, 2). Here, we show that coherence in such devices can survive much longer, and provide a detailed understanding of the measured nuclear-spin-induced decoherence. At fields above a few hundred millitesla, the coherence time measured using a single-pulse spin echo is 30 μs. At lower fields, the echo first collapses, but then revives at times determined by the relative Larmor precession of different nuclear species. This behaviour was recently predicted3, 4, and can, as we show, be quantitatively accounted for by a semiclassical model for the dynamics of electron and nuclear spins. Using a multiple-pulse Carr–Purcell–Meiboom–Gillecho sequence, the decoherence time can be extended to more than 200 μs, an improvement by two orders of magnitude compared with previous measurements1, 2, 5.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1856.html

Andreev bound states in supercurrent-carrying carbon nanotubes revealed

J-D. Pillet, C. H. L. Quay, P. Morfin, C. Bena, A. Levy Yeyati & P. Joyez

Carbon nanotubes (CNTs) are not intrinsically superconducting but they can carry a supercurrent when connected to superconducting electrodes1, 2, 3, 4. This supercurrent is mainly transmitted by discrete entangled electron–hole states confined to the nanotube, called Andreev bound states (ABS). These states are a key concept in mesoscopic superconductivity as they provide a universal description of Josephson-like effects in quantum-coherent nanostructures (for example molecules, nanowires, magnetic or normal metallic layers) connected to superconducting leads5. We report here the first tunnelling spectroscopy of individually resolved ABS, in a nanotube–superconductor device. Analysing the evolution of the ABS spectrum with a gate voltage, we show that the ABS arise from the discrete electronic levels of the molecule and that they reveal detailed information about the energies of these levels, their relative spin orientation and the coupling to the leads. Such measurements hence constitute a powerful new spectroscopic technique capable of elucidating the electronic structure of CNT-based devices, including those with well-coupled leads. This is relevant for conventional applications (for example, superconducting or normal transistors, superconducting quantum interference devices3 (SQUIDs)) and quantum information processing (for example, entangled electron pair generation6, 7, ABS-based qubits8). Finally, our device is a new type of d.c.-measurable SQUID.

http://www.nature.com/nphys/journal/v6/n12/full/nphys1811.html

Coherent electron–nuclear coupling in oligothiophene molecular wires

Jascha Repp, Peter Liljeroth & Gerhard Meyer

In molecular electronics individual molecules serve as electronic devices. In these systems, electron–vibron (e–ν) coupling can be expected to lead to new physical phenomena and potential device functions1, 2, 3. In previous studies of molecular wires, the e–ν coupling occurred as a result of the well-known Franck–Condon principle, for which the Born–Oppenheimer approximation holds. This means that after a vibronic excitation, the electrons and the vibrations evolve independently from each other. Here we show that this simple picture changes markedly when two electronic levels in a molecule are coupled by a molecular vibration4, 5. In molecular wires we observe a non-Born–Oppenheimer regime, for which a coherent coupling of electronic and nuclear motion emerges6. This phenomenon should occur in all systems with strong electron–vibration coupling and an electronic level spacing of the order of vibrational energies. The coherent coupling of electronic and nuclear motion could be used to implement mechanical control of electron transport in molecular electronics.

http://www.nature.com/nphys/journal/v6/n12/full/nphys1802.html

Vibrational and electronic heating in nanoscale junctions

Daniel R. Ward, David A. Corley, James M. Tour & Douglas Natelson

Understanding and controlling the flow of heat is a major challenge in nanoelectronics. When a junction is driven out of equilibrium by light or the flow of electric charge, the vibrational and electronic degrees of freedom are, in general, no longer described by a single temperature1, 2, 3, 4, 5, 6. Moreover, characterizing the steady-state vibrational and electronic distributions in situ is extremely challenging. Here, we show that surface-enhanced Raman emission may be used to determine the effective temperatures for both the vibrational modes and the electrons in the current in a biased metallic nanoscale junction decorated with molecules7. Molecular vibrations show mode-specific pumping by both optical excitation8 and d.c. current9, with effective temperatures exceeding several hundred kelvin. Anti-Stokes electronic Raman emission10, 11 indicates that the effective electronic temperature at bias voltages of a few hundred millivolts can reach values up to three times the values measured when there is no current. The precise effective temperatures are model-dependent, but the trends as a function of bias conditions are robust, and allow direct comparisons with theories of nanoscale heating.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2010.240.html

Scanning probe microscopy: STM hits the fast lane

Alexander Ako Khajetoorians & André Kubetzka

A pump–probe approach allows the relaxation times of single spins to be measured.

http://www.nature.com/nnano/journal/v5/n12/full/nnano.2010.243.html

Electroluminescence from a single nanotube–molecule–nanotube junction

Christoph W. Marquardt, Sergio Grunder, Alfred Bl strokeaszczyk, Simone Dehm, Frank Hennrich, Hilbert v. Löhneysen, Marcel Mayor & Ralph Krupke

The positioning of single molecules between nanoscale electrodes1, 2, 3, 4, 5, 6, 7, 8 has allowed their use as functional units in electronic devices. Although the electrical transport in such devices has been widely explored, optical measurements have been restricted to the observation of electroluminescence from nanocrystals and nanoclusters9, 10 and from molecules in a scanning tunnelling microscope setup11, 12. In this Letter, we report the observation of electroluminescence from the core of a rod-like molecule between two metallic single-walled carbon nanotube electrodes forming a rigid solid-state device. We also develop a simple model to explain the onset voltage for electroluminescence. These results suggest new characterization and functional possibilities, and demonstrate the potential of carbon nanotubes for use in molecular electronics.

http://www.nature.com/nnano/journal/v5/n12/full/nnano.2010.230.html

Kondo effect of a Co atom on Cu(111) in contact with an iron tip

N. Néel, J. Kröger*, and R. Berndt

Single Co atoms, which exhibit a Kondo effect on Cu(111), are contacted with Cu and Fe tips in a low-temperature scanning tunneling microscope. With Fe tips, the Kondo effect persists with the Abrikosov-Suhl resonance significantly broadened. In contrast, for Cu-covered W tips, the resonance width remains almost constant throughout the tunneling and contact ranges. The distinct changes in the line width are interpreted in terms of modifications of the Co d state occupation owing to hybridization with the tip apex atoms.

http://prb.aps.org/abstract/PRB/v82/i23/e235303

Controlling Electron-Phonon Interactions in Graphene at Ultrahigh Carrier Densities

Dmitri K. Efetov and Philip Kim

We report on the temperature dependent electron transport in graphene at different carrier densities n. Employing an electrolytic gate, we demonstrate that n can be adjusted up to 4×1014 cm-2 for both electrons and holes. The measured sample resistivity ρ increases linearly with temperature T in the high temperature limit, indicating that a quasiclassical phonon distribution is responsible for the electron scattering. As T decreases, the resistivity decreases more rapidly following ρ(T)∼T4. This low temperature behavior can be described by a Bloch-Grüneisen model taking into account the quantum distribution of the two-dimensional acoustic phonons in graphene. We map out the density dependence of the characteristic temperature ΘBG defining the crossover between the two distinct regimes, and show that, for all n, ρ(T) scales as a universal function of the normalized temperature T/ΘBG.

http://prl.aps.org/abstract/PRL/v105/i25/e256805

Dec. 2. - Dec. 8. (2010)

Válogatta: Balogh Zoltán

Low Energy Coherent Transport in Metallic Carbon Nanotube Junctions

Authors: A.A. Maarouf, E.J. Mele

We study the low-energy electronic properties of a junction made of two crossed metallic carbon nanotubes of general chiralities. We derive a tight binding tunneling matrix element that couples low-energy states on the two tubes, which allows us to calculate the contact conductance of the junction. We find that the intrinsic asymmetries of the junction cause the forward and backward hopping probabilities from one tube to another to be different. This defines a zero-field Hall conductance for the junction, which we find to scale inversely with the junction contact conductance. Through a systematic study of the dependence of the junction conductance on different junction parameters, we find that the crossing angle is the dominant factor which determines the magnitude of the conductance.

http://arxiv.org/abs/1012.0355

Imaging charge density fluctuations in graphene using Coulomb blockade spectroscopy

Authors: A. Deshpande, W. Bao, H. Zhang, Z. Zhao, C.N. Lau, B.J. LeRoy

Using scanning tunneling microscopy, we have imaged local charge density fluctuations in monolayer graphene. By placing a small gold nanoparticle on the end of the STM tip, a charge sensor is created. By raster scanning the tip over the surface and using Coulomb blockade spectroscopy, we map the local charge on the graphene. We observe a series of electron and hole doped puddles with a characteristic length scale of about 20 nm. Theoretical calculations for the correlation length of the puddles based on the number of impurities are in agreement with our measurements.

http://arxiv.org/abs/1012.0374

Anomalous Josephson current via Majorana bound states in topological insulators

Authors: P. A. Ioselevich, M. V. Feigel'man

We propose a setup involving Majorana bound states (MBS) hosted by a vortex on a superconducting surface of a 3D Topological Insulator (TI). We consider a narrow channel drilled across a TI slab with both sides covered by s-wave superconductor. In the presence of a vortex pinned to such a channel, it acts as a ballistic nanowire connecting the superconducting surfaces, with a pair of MBS localized in it. The energies of the MBS possess a 4\pi-periodic dependence on the superconductive phase difference \phi between the surfaces. It results in the appearence of an anomalous term in the current-phase relation, I_a(\phi) for the supercurrent flowing along the channel between the superconductive surfaces. We have calculated the shape of the 4\pi-periodic function I_a(\phi), as well as the dependence of its amplitude on temperature and system parameters.

http://arxiv.org/abs/1012.0407

Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment'

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

Graphene stacked in a Bernal configuration (60 degrees relative rotations between sheets) differs electronically from isolated graphene due to the broken symmetry introduced by interlayer bonds forming between only one of the two graphene unit cell atoms. A variety of experiments have shown that non-Bernal rotations restore this broken symmetry; consequently, these stacking varieties have been the subject of intensive theoretical interest. Most theories predict substantial changes in the band structure ranging from the development of a Van Hove singularity and an angle dependent electron localization that causes the Fermi velocity to go to zero as the relative rotation angle between sheets goes to zero. In this work we show by direct measurement that non-Bernal rotations preserve the graphene symmetry with only a small perturbation due to weak effective interlayer coupling. We detect neither a Van Hove singularity nor any significant change in the Fermi velocity. These results suggest significant problems in our current theoretical understanding of the origins of the band structure of this material.

http://arxiv.org/abs/1012.0460

Controlling non-Abelian statistics of Majorana fermions in quasi-one-dimensional wire networks

Authors: Jay D. Sau, D. J. Clarke, S. Tewari

Under appropriate external conditions a spin-orbit coupled semiconductor in proximity to a s-wave superconductor can be in a topological superconducting (TS) phase. In the topological phase, various defects of the order parameter trap zero energy excitations called Majorana bound states. For a semiconductor nanowire, the relevant defects are the two ends of the wire, and each traps a localized zero energy excitation. A network of such nanowires with 1D TS segments separated by non-topological superconducting (NTS) regions has been proposed recently as a platform for topological quantum computation (TQC). For TQC, a primary requirement is that the spatial positions of the Majorana excitations can be pairwise exchanged, and that they follow non-Abelian statistics under such an exchange. Alicea et al. have shown how non-Abelian statistics arises on the wire network by approximately mapping the system to a 1D lattice model of fermions. Here we present an alternative approach to realizing the non-Abelian statistics on a wire-network based on gate-controlled tunneling between Majorana fermions. The non-Abelian phases realized by this scheme are found to be uniquely determined by the microscopic tunneling parameters of a given wire network and can be computed using the Heisenberg equations of motion.

http://arxiv.org/abs/1012.0561

Energy-efficient mixed mode switching of a multiferroic nanomagnet for logic and memory

Authors: Kuntal Roy, Supriyo Bandyopadhyay, Jayasimha Atulasimha

In magnetic memory and logic devices, a magnet's magnetization is usually flipped with a spin polarized current delivering a spin transfer torque (STT). This mode of switching consumes too much energy and considerable energy saving can accrue from using a multiferroic nanomagnet switched with a combination of STT and mechanical stress generated with a voltage (VGS). The VGS mode consumes less energy than STT, but cannot rotate magnetization by more than 90?, so that a combination of the two modes is needed for energy-efficient switching.

http://arxiv.org/abs/1012.0819

Switching energy-delay of all-spin logic devices

Authors: Behtash Behin-Aein, Angik Sarkar, Srikant Srinivasan, Supriyo Datta

The need to find low power alternatives to digital electronic circuits has led to increasing interest in alternative switching schemes like the magnetic quantum cellular automata(MQCA) that store information in nanomagnets which communicate through their magnetic fields. A recent proposal called all spin logic (ASL) proposes to communicate between nanomagnets using spin currents which are spatially localized and can be conveniently routed. The objective of this paper is to present a model for ASL devices that is based on established physics and is benchmarked against available experimental data and to use it to investigate switching energy-delay of ASL devices.

http://arxiv.org/abs/1012.0861

Quantum noise of an electromagnetically controlled two level system

Authors: Ching-Kit Chan, L. J. Sham

Coherent control of a spin is limited by both the decoherence due to coupling with the environment and noise coming from the quantized control. The quantum noise study of this system is particularly important in fault tolerant quantum computation where a very high fidelity is demanded. Here, we present a time evolution study of a two level system (TLS) interacting with a laser pulse and the electromagnetic vacuum based on the multimode Jaynes-Cummings model. We develop a diagrammatic formalism in which one can easily identify the coherent Rabi oscillation of the TLS and its relaxation from corresponding diagrams. In the small time limit ($t\ll T_1$), where the noise level is small but still an issue to fault tolerant quantum computing, this method gives a quantitative evaluation of the quantum noise of the TLS under an optical control with an arbitrary pulse shape. Furthermore, this approach can be naturally extended from the Markovian to the non-Markovian regime, resulting in dynamics different from that obtained in the optical Bloch analysis. All these calculations are done without any stochastic assumption.

http://arxiv.org/abs/1012.1051

Ballistic transport at room temperature in micrometer size multigraphene

Authors: S. Dusari, J. Barzola-Quiquia, P. Esquinazi, N. García

The intrinsic values of the carriers mobility and density of the graphene layers inside graphite, the well known structure built on these layers in the Bernal stacking configuration, are not well known mainly because most of the research was done in rather bulk samples where lattice defects hide their intrinsic values. By measuring the electrical resistance through microfabricated constrictions in micrometer small graphite flakes of a few tens of nanometers thickness we studied the ballistic behavior of the carriers. We found that the carriers' mean free path is micrometer large with a mobility $\mu \simeq 6 \times 10^6 $cm$^2$/Vs and a carrier density $n \simeq 7 \times 10^8 $cm$^{-2}$ per graphene layer at room temperature. These distinctive transport and ballistic properties have important implications for understanding the values obtained in single graphene and in graphite as well as for implementing this last in nanoelectronic devices.

http://arxiv.org/abs/1012.1100

Enhanced DNA sequencing performance through edge-hydrogenation of graphene electrodes

Authors: Yuhui He, Ralph H. Scheicher, Anton Grigoriev, Rajeev Ahuja, Shibing Long, ZongLiang Huo, Ming Liu

We propose using graphene electrodes with hydrogenated edges for solid-state nanopore-based DNA sequencing, and perform molecular dynamics simulations in conjunction with electronic transport calculations to explore the potential merits of this idea. The results of our investigation show that, compared to the unhydrogenated system, edge-hydrogenated graphene electrodes facilitate the temporary formation of H-bonds with suitable atomic sites in the translocating DNA molecule. As a consequence, the average conductivity is drastically raised by about 3 orders of magnitude while exhibiting significantly reduced statistical variance. We have furthermore investigated how these results are affected when the distance between opposing electrodes is varied and have identified two regimes: for narrow electrode separation, the mere hindrance due to the presence of protruding hydrogen atoms in the nanopore is deemed more important, while for wider electrode separation, the formation of H-bonds becomes the dominant effect. Based on these findings, we conclude that hydrogenation of graphene electrode edges represents a promising approach to reduce the translocation speed of DNA through the nanopore and substantially improve the accuracy of the measurement process for whole-genome sequencing.

http://arxiv.org/abs/1012.0031

Weak magnetism in insulating and superconducting cuprates

G. M. De Luca,*, G. Ghiringhelli, M. Moretti Sala, S. Di Matteo, M. W. Haverkort, H. Berger, V. Bisogni, J. C. Cezar, N. B. Brookes, and M. Salluzzo

X-ray magnetic circular dichroism provides evidence of an out-of-plane spin moment in undoped and doped cuprates. In La2CuO4 this moment is related to the canting of the antiferromagnetically ordered Cu2+ spins caused by the Dzyaloshinskii-Moriya interaction within the CuO2 planes. This canting gives rise to the well-known weak ferromagnetism in high magnetic fields. We find a similar behavior in doped compounds, both in the normal and in the superconducting state, but with a different temperature dependence typical of paramagnetic systems. This result suggests that, together with short-range in-plane antiferromagnetic correlations, the Dzyaloshinskii-Moriya interaction survives up to optimal doping and in the superconducting state.

http://prb.aps.org/abstract/PRB/v82/i21/e214504

Spin-triplet supercurrent through inhomogeneous ferromagnetic trilayers

Mohammad Alidoust and Jacob Linder

Motivated by a recent experiment [J. W. A. Robinson, J. D. S. Witt, and M. G. Blamire, Science 329, 5987 (2010)], we report here the possibility of establishing a long-range spin-triplet supercurrent through an inhomogeneous ferromagnetic region consisting of a Ho∣Co∣Ho trilayer sandwiched between conventional s-wave superconducting leads. We utilize a full numerical solution in the diffusive regime of transport and study the behavior of the supercurrent for various experimentally realistic configurations of the ferromagnetic trilayer. We obtain qualitatively very good agreement with experimental data regarding the behavior of the supercurrent as a function of the width of the Co layer, LCo. Moreover, we find a synthesis of 0-π oscillations with superimposed rapid oscillations when varying the width of the Ho layers symmetrically, which pertain specifically to the spiral magnetization texture in Ho. Although we are not able to reproduce the anomalous sharp peaks in the supercurrent vs Ho-layer thickness observed experimentally in this regime, the results obtained are quite sensitive to the exact magnetization profile in the Ho layers. This might be the reason for the discrepancy between our results and the experimental reported data for this particular aspect. We also investigate the supercurrent in a system where the intrinsically inhomogeneous Ho ferromagnets are replaced with domain-wall ferromagnets and find similar behavior as in the Ho∣Co∣Ho case. Furthermore, we propose magnetic Josephson junctions including only a domain-wall ferromagnet and a homogeneous ferromagnetic layer. The hybrid structure not only is simple regarding the magnetization profile but also offers a tunable long-range spin-triplet supercurrent. Finally, we discuss some experimental aspects of our findings.

http://prb.aps.org/abstract/PRB/v82/i22/e224504

Imaging the electron density in solids by using multi-Brillouin-zone angle resolved photoelectron spectroscopy

W. S. Jung, C. S. Leem, Chul Kim, S. R. Park, S. Y. Park, B. J. Kim, E. Rotenberg, and C. Kim

We propose a method to measure the electron density in the real space by using angle resolved photoelectron spectroscopy (ARPES). By expanding the wave function in terms of Wannier functions, multi-Brillouin-zone ARPES data contains information on the coefficients of the Wannier function. It is shown, in the case of noninteracting electrons, that ARPES spectral functions in different Brillouin zones are related to the absolute value of the Fourier components of the initial states. In addition, the phases, which are a pseudospin in the graphene, of these components are shown to be real numbers in the function of ARPES intensity. We simulate ARPES data from tight-binding model to obtain phase information. This information combined with a proper consideration of the electronic potential can be used to construct the real-space wave function.

http://prb.aps.org/abstract/PRB/v82/i23/e235105

Spin-orbit coupling and phase coherence in InAs nanowires

S. Estévez Hernández, M. Akabori, K. Sladek, Ch. Volk, S. Alagha, H. Hardtdegen, M. G. Pala, N. Demarina, D. Grützmacher, and Th. Schäpers

We investigated the magnetotransport of InAs nanowires grown by selective-area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation in the magnetic field or the backgate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length lϕ is determined. At the lowest temperatures lϕ is found to be at least 300 nm while for temperatures exceeding 2 K a monotonous decrease in lϕ with temperature is observed. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctuations. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the experimental data to simulations based on a recursive two-dimensional Green’s-function approach a spin-orbit scattering length of approximately 70 nm was extracted, indicating the presence of strong spin-orbit coupling.

http://prb.aps.org/abstract/PRB/v82/i23/e235303

Nanomechanical effects in an Andreev quantum dot

I. A. Sadovskyy, G. B. Lesovik, T. Jonckheere, and T. Martin

We consider a quantum dot with mechanical degrees of freedom which is coupled to superconducting electrodes. A Josephson current is generated by applying a phase difference. In the absence of coupling to vibrations, this setup was previously proposed as a detector of magnetic flux and we wish here to address the effect of the phonon coupling to this detection scheme. We compute the charge on the quantum dot and determine its dependence on the phase difference in the presence of phonon coupling and Coulomb interaction. This allows to identify regions in parameter space with the highest charge to phase sensitivity, which are relevant for flux detection. Further insight about the interplay of such couplings and subsequent entanglement properties between electron and phonon degrees of freedom are gained by computing the von Neumann entropy.

http://prb.aps.org/abstract/PRB/v82/i23/e235310

Disentangling Cooper-pair formation above the transition temperature from the pseudogap state in the cuprates

Takeshi Kondo, Yoichiro Hamaya, Ari D. Palczewski, Tsunehiro Takeuchi, J. S. Wen, Z. J. Xu, Genda Gu, Jörg Schmalian & Adam Kaminski

The discovery of the pseudogap in the cuprates1, 2, 3 created significant excitement as it was believed to be a signature of pairing4, in some cases above room temperature. Indeed, a number of experiments detected phase-fluctuating superconductivity above the transition temperature Tc (refs 5, 6, 7 8, 9). However, several recent experiments reported that the pseudogap and superconducting state are characterized by different energy scales10, 11, 12, 13, 14, and probably compete with each other15, 16, leaving open the question of whether the pseudogap is caused by pair formation. Here we report the discovery of a spectroscopic signature of pair formation and demonstrate that in a region commonly referred to as the pseudogap, two distinct states coexist: one that is due to pair formation and persists to an intermediate temperature Tpair<T* and a second—the ‘proper’ pseudogap—characterized by the loss of spectral weight and anomalies in transport properties that extends up to T*. Tpair has a value around 120–150 K even for materials with very different Tc values and it probably sets a limit on the highest attainable Tc in the cuprates.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1851.html

Direct observation of single-charge-detection capability of nanowire field-effect transistors

J. Salfi, I. G. Savelyev, M. Blumin, S. V. Nair & H. E. Ruda

A single localized charge can quench the luminescence of a semiconductor nanowire1, but relatively little is known about the effect of single charges on the conductance of the nanowire. In one-dimensional nanostructures embedded in a material with a low dielectric permittivity, the Coulomb interaction and excitonic binding energy are much larger than the corresponding values when embedded in a material with the same dielectric permittivity2, 3. The stronger Coulomb interaction is also predicted to limit the carrier mobility in nanowires4. Here, we experimentally isolate and study the effect of individual localized electrons on carrier transport in InAs nanowire field-effect transistors, and extract the equivalent charge sensitivity. In the low carrier density regime, the electrostatic potential produced by one electron can create an insulating weak link in an otherwise conducting nanowire field-effect transistor, modulating its conductance by as much as 4,200% at 31 K. The equivalent charge sensitivity, 4 × 10−5 e Hz−1/2 at 25 K and 6 × 10−5 e Hz−1/2 at 198 K, is orders of magnitude better than conventional field-effect transistors5 and nanoelectromechanical systems6, 7, and is just a factor of 20–30 away from the record sensitivity for state-of-the-art single-electron transistors operating below 4 K (ref. 8). This work demonstrates the feasibility of nanowire-based single-electron memories9 and illustrates a physical process of potential relevance for high performance chemical sensors10, 11. The charge-state-detection capability we demonstrate also makes the nanowire field-effect transistor a promising host system for impurities (which may be introduced intentionally or unintentionally) with potentially long spin lifetimes12, 13, because such transistors offer more sensitive spin-to-charge conversion readout than schemes based on conventional field-effect transistors13.

http://www.nature.com/nnano/journal/v5/n10/full/nnano.2010.180.html

Low-Voltage p- and n-Type Organic Self-Assembled Monolayer Field Effect Transistors

Michael Novak, Alexander Ebel, Timo Meyer-Friedrichsen, Abdesselam Jedaa, Benito F. Vieweg, Guang Yang, Kislon Voitchovsky, Francesco Stellacci, Erdmann Spiecker, Andreas Hirsch, and Marcus Halik

We report on p- and n-type organic self-assembled monolayer field effect transistors. On the base of quaterthiophene and fullerene units, multifunctional molecules were synthesized, which have the ability to self-assemble and provide multifunctional monolayers. The self-assembly approach, based on phosphonic acids, is very robust and allows the fabrication of functional devices even on larger areas. The p- and n-type transistor devices with only one molecular active layer were demonstrated for transistor channel lengths up to 10 μm. The monolayer composition is proven by electrical experiments and by high-resolution transmission electron microscopy, electron energy loss spectroscopy, XPS, and AFM experiments. Because of the molecular design and the contribution of isolating alkyl chains to the hybrid dielectric, our devices operate at low supply voltages (−4 V to +4 V), which is a key requirement for practical use and simplifies the integration in standard applications. The monolayer devices operate in ambient air and show hole and electron mobilities of 10−5 cm2/(V s) and 10−4 cm2/(V s) respectively. In particular the n-type operation of self-assembled monolayer transistors has not been reported before. Hereby, structure−property relations of the SAMs have been studied. Furthermore an approach to protect the sensitive C60 from immediate degradation within the molecular design is provided.

http://pubs.acs.org/doi/abs/10.1021/nl103200r

Single-Molecule Mapping of Long-range Electron Transport for a Cytochrome b562 Variant

Eduardo Antonio Della Pia, Qijin Chi, D. Dafydd Jones, J. Emyr Macdonald, Jens Ulstrup, and Martin Elliott

Cytochrome b562 was engineered to introduce a cysteine residue at a surface-exposed position to facilitate direct self-assembly on a Au(111) surface. The confined protein exhibited reversible and fast electron exchange with a gold substrate over a distance of 20 Å between the heme redox center and the gold surface, a clear indication that a long-range electron-transfer pathway is established. Electrochemical scanning tunneling microscopy was used to map electron transport features of the protein at the single-molecule level. Tunneling resonance was directly imaged and apparent molecular conductance was measured, which both show strong redox-gated effects. This study has addressed the first case of heme proteins and offered new perspectives in single-molecule bioelectronics.

http://pubs.acs.org/doi/abs/10.1021/nl103334q

Graphene Edge from Armchair to Zigzag: The Origins of Nanotube Chirality?

Yuanyue Liu, Alex Dobrinsky, and Boris I. Yakobson

The energy of an arbitrary graphene edge, from armchair (A) to zigzag (Z) orientation, is derived in analytical form. It contains a “chemical phase shift” determined by the chemical conditions at the edge. Direct atomistic computations support the universal nature of the relationship, definitive for graphene formation, and shapes of the voids or ribbons. It has further profound implications for nanotube chirality selection and possibly control by chemical means, at the nucleation stage.

http://prl.aps.org/abstract/PRL/v105/i23/e235502

Nov. 26. - Dec. 1. (2010)

Válogatta: Scherübl Zoltán

Functionalized Graphene for High Performance Two-dimensional Spintronics Devices

Authors: Linze Li, Rui Qin, Hong Li, Qihang Liu, Guangfu Luo, Jing Lu, Zhengxiang Gao

Using first-principles calculations, we explore the possibility of functionalized graphene as high performance two-dimensional spintronics device. Graphene functionalized with O on one side and H on the other side in the chair conformation is found to be a ferromagnetic metal with a spin-filter efficiency up to 85% at finite bias. The ground state of graphene semi-functionalized with F in the chair conformation is an antiferromagnetic semiconductor, and we construct a magnetoresistive device from it by introducing a magnetic field to stabilize its ferromagnetic metallic state. The resulting room-temperature magnetoresistance is up to 5400%, which is one order of magnitude larger than the available experimental values.

arXiv:1011.6014v1

The competition between superconductivity and ferromagnetism in small metallic grains: thermodynamic properties'

Authors: K. Van Houcke, Y. Alhassid, S. Schmidt, S. M. A. Rombouts

We study the thermodynamic properties of a small superconducting metallic grain using a quantum Monte Carlo method. The grain is described by the universal Hamiltonian, containing pairing and ferromagnetic exchange correlations. In particular, we study how the thermodynamic signatures of pairing correlations are affected by the spin exchange interaction. We find the exchange interaction effects to be qualitatively different in the BCS and fluctuation-dominated regimes of pairing correlations.

arXiv:1011.5421v1

Molecule States in a Gate Tunable Graphene Double Quantum Dot

Authors: L. J. Wang, H. O. Li, Z. Su, T. Tu, G. Cao, C. Zhou, X. J. Hao, G. C. Guo, G. P. Guo

We have measured a graphene double quantum dot device with multiple electrostatic gates that are used to enhance control to investigate it. At low temperatures the transport measurements reveal honeycomb charge stability diagrams which can be tuned from weak to strong interdot tunnel coupling regimes. We precisely extract a large interdot tunnel coupling strength for this system allowing for the observation of tunnel-coupled molecular states extending over the whole double dot. This clean, highly controllable system serves as an essential building block for quantum devices in a nuclear-spin-free world.

arXiv:1011.5347v1

Fine structure of "zero-mode" Landau levels in HgTe/HgCdTe quantum wells

Authors: M. Orlita, K. Masztalerz, C. Faugeras, M. Potemski, E. G. Novik, C. Brüne, H. Buhmann, L. W. Molenkamp

HgTe/HgCdTe quantum wells with the inverted band structure have been probed using far infrared magneto-spectroscopy. Realistic calculations of Landau level diagrams have been performed to identify the observed transitions. Investigations have been greatly focused on the magnetic field dependence of the peculiar pair of "zero-mode" Landau levels which characteristically split from the upper conduction and bottom valence bands, and merge under the applied magnetic field. The observed avoided crossing of these levels is tentatively attributed to the bulk inversion asymmetry of zinc blend compounds.

arXiv:1011.5233v1

Magneto-transport of large CVD-grown graphene

Authors: Eric Whiteway, Victor Yu, Josianne Lefebvre, Robert Gagnon, Michael Hilke

We present magnetoresistance measurements on large scale monolayer graphene grown by chemical vapor deposition (CVD) on copper. The graphene layer was transferred onto SiO2/Si via PMMA and thermal release tape for transport measurements. The resulting centimeter-sized graphene samples were measured at temperatures down to 30mK in a magnetic field. We observe a very sharp peak in resistance at zero field, which is well fitted by weak localization theory. The samples exhibit conductance fluctuations symmetric in field, which are attributed to ensemble averaged conductance fluctuations due to large scale inhomogeneities consistent with the grain boundaries of copper during the CVD growth.

arXiv:1011.5712v1

Strongly Modified Plasmon-Matter Interaction with Mesoscopic Quantum Emitters

Authors: Mads Lykke Andersen, Søren Stobbe, Anders Søndberg Sørensen, Peter Lodahl

Semiconductor quantum dots (QDs) provide an essential link between light and matter in emerging fields such as light-harvesting, all-solid-state quantum communication, and quantum computing. QDs are excellent single-photon sources and can store quantum bits for extended periods making them promising interconnects between light and matter in integrated quantum information networks. To this end the light-matter interaction strength must be strongly enhanced using nanophotonic structures such as photonic crystal cavities and waveguides or plasmonic nanowires. So far it has been assumed that QDs can be treated just like atomic photon emitters where the spatial properties of the wavefunction can be safely ignored. Here we demonstrate that the point-emitter description for QDs near plasmonic nanostructures breaks down. We observe that the QDs can excite plasmons eight times more efficiently depending on their orientation due to their mesoscopic character. Either enhancement or suppresion of the rate of plasmon excitation is observed depending on the geometry of the plasmonic nanostructure in full agreement with a new theory. This discovery has no equivalence in atomic systems and paves the way for novel nanophotonic devices that exploit the extended size of QDs as a resource for increasing the light-matter interaction strength.

arXiv:1011.5669v1

Andreev reflection from non-centrosymmetric superconductors and Majorana bound state generation in half-metallic ferromagnets

Authors: Mathias Duckheim, Piet W. Brouwer

We study Andreev reflection at an interface between a half metal and a superconductor with spin-orbit interaction. While the absence of minority carriers in the half metal makes singlet Andreev reflection impossible, the spin-orbit interaction gives rise to triplet Andreev reflection, i.e., the reflection of a majority electron into a majority hole or vice versa. As an application of our calculation, we consider a thin half metal film or wire laterally attached to a superconducting contact. If the half metal is disorder free, an excitation gap is opened that is proportional to the spin-orbit interaction strength in the superconductor. For electrons with energy below this gap a lateral half-metal--superconductor contact becomes a perfect triplet Andreev reflector. We show that the system supports localized Majorana end states in this limit.

arXiv:1011.5839v1

Quantitative description of Josephson-like tunneling in $\nu_T=1$ quantum Hall bilayers

Authors: Timo Hyart, Bernd Rosenow

At total filling factor $\nu_T=1$, interlayer phase coherence in quantum Hall bilayers can result in a tunneling anomaly resembling the Josephson effect in the presence of strong fluctuations. The most robust experimental signature of this effect is a strong enhancement of the tunneling conductance at small voltages. The height and width of the conductance peak depend strongly on the area and tunneling amplitude of the samples, applied parallel magnetic field and temperature. We find that the tunneling experiments are in quantitative agreement with a theory which treats fluctuations due to meron excitations phenomenologically and takes tunneling into account perturbatively. We also discuss the qualitative changes caused by larger tunneling amplitudes, and provide a possible explanation for recently observed critical currents in counterflow geometry.

arXiv:1011.5684v1

Dissipative dynamics of two-qubit system: four-level lasing

Authors: E. A. Temchenko, S. N. Shevchenko, A. N. Omelyanchouk

The dissipative dynamics of a two-qubit system is studied theoretically. We make use of the Bloch-Redfield formalism which explicitly includes the parameter-dependent relaxation rates. We consider the case of two flux qubits, when the controlling parameters are the partial magnetic fluxes through the qubits' loops. The strong dependence of the inter-level relaxation rates on the controlling magnetic fluxes is demonstrated for the realistic system. This allows us to propose several mechanisms for lasing in this four-level system.

arXiv:1011.5598v1

Current noise and Coulomb effects in superconducting contacts

Authors: A.V. Galaktionov, A.D. Zaikin

We derive an effective action for contacts between superconducting terminals with arbitrary transmission distribution of conducting channels. In the case of normal-superconducting (NS) contacts we evaluate interaction correction to Andreev conductance and demonstrate a close relation between Coulomb effects and shot noise in these systems. In the case of superconducting (SS) contacts we derive the electron-electron interaction correction to the Josephson current. At $T=0$ both corrections are found to vanish for fully transparent NS and SS contacts indicating the absence of Coulomb effects in this limit.

arXiv:1011.5571v1

Effects of magnetic field and transverse anisotropy on full counting statistics in single-molecule magnet

Authors: Hai-Bin Xue, Y.-H. Nie, Z.-J. Li, J.-Q. Liang We have studied theoretically the full counting statistics of electron transport through a single-molecule magne (SMM) with an arbitrary angle between the applied magnetic field and the SMM's easy axis above the sequential tunneling threshold, since the angle $\theta$ cannot be controlled in present-day SMM experiments. In the absence of the small transverse anisotropy, when the coupling of the SMM with the incident-electrode is stronger than that with the outgoing-electrode, i.e., $\Gamma_{L}/\Gamma _{R}\gg1$, the maximum peak of shot noise firstly increase and then decrease with increasing $\theta$ from $0$ to $0.5\pi$. In particular, the shot noise can reach up to super-Poissonian value from sub-Poissonian when considering the small transverse anisotropy. For $\Gamma_{L}/\Gamma_{R}\ll1$, the maximum peaks of the shot noise and skewness can be reduced from super-Poissonian to sub-Poissonian value with increasing $\theta$ from $0$ to $0.5\pi$; and the super-Poissonian behavior of the skewness is more sensitive to the small $\theta$ than shot noise, which is suppressed when taking into account the small transverse anisotropy. These characteristics of shot noise can be qualitatively attributed to the competition between the fast and slow transport channels. The predictions regarding of the $\theta$-dependence of high order current cumulants are very interesting for better understanding electron transport through SMM, and permit experimental tests in the near future.

arXiv:1011.5546v1

Electron-Phonon Interactions in Bilayer Graphene: A First Principles Approach

Authors: K. M. Borysenko, J. T. Mullen, X. Li, Y. G. Semenov, J. M. Zavada, M. Buongiorno Nardelli, K. W. Kim

Density functional perturbation theory is used to analyze electron-phonon interaction in bilayer graphene. The results show that phonon scattering in bilayer graphene bears more resemblance with bulk graphite than monolayer graphene. In particular, electron-phonon scattering in the lowest conduction band is dominated by six lowest (acoustic and acoustic-like) phonon branches with only minor contributions from optical modes. The total scattering rate at low/moderate electron energies can be described by a simple two-phonon model in the deformation potential approximation with effective constants Dac $\approx$ 15 eV and Dop $\approx 2.8 \times 108$ eV/cm for acoustic and optical phonons, respectively. With much enhanced acoustic phonon scattering, the low field mobility of bilayer graphene is expected to be significantly smaller than that of monolayer graphene.

arXiv:1011.5521v1

Interedge magnetic coupling in transition-metal terminated graphene nanoribbons

Authors: Yan Wang, Hai-Ping Cheng

The magnetic structures and interedge magnetic couplings of Fe, Co and Ni transition-metal terminated graphene nanoribbons with zigzag (ZGNR) and armchair (AGNR) edges are studied by first-principles calculations. Fe-ZGNR is found to show antiferromagnetic (AF) coupling between two edges, while the interedge coupling of Co-ZGNR is ferromagnetic (FM). For Fe-AGNRs and Co-AGNRs, increasing the interedge distance we follow oscillatory transitions from FM to AF coupling with a period of about 3.7 {\AA}. The damped oscillatory behavior indicates a Ruderman-Kittel-Kasuya-Yosida type interedge magnetic coupling and the oscillation period is determined by the critical spanning vector which connects two inequivalent Dirac points in the graphene Brillouin zone. The two edges in Ni-ZGNR are decoupled independent of the ribbon width and Ni-AGNRs are found to be nonmangetic.

arXiv:1011.6356v1

Energetics and stability of vacancies in carbon nanotubes

Authors: José Eduardo Padilha, Rodrigo Garcia Amorim, Alexandre Reily Rocha, Antônio José Roque da Silva, Adalberto Fazzio

In this work we present ab initio calculations of the formation energies and stability of different types of multi-vacancies in carbon nanotubes. We demonstrate that, as in the case of graphene, the reconstruction of the defects has drastic effects on the energetics of the tubes. In particular, the formation of pentagons eliminates the dangling bonds thus lowering the formation energy. This competition leads to vacancies having an even number of carbon atoms removed to be more stable. Finally the appearance of magic numbers indicating more stable defects can be represented by a model for the formation energies that is based on the number of dangling bonds of the unreconstructed system, the pentagons and the relaxation of the final form of the defect formed after the relaxation.

arXiv:1011.6316v1

Quantum interference effects in a system of two tunnel point-contacts in the presence of single scatterer: simulation of a double-tip STM experiment

Authors: N.V. Khotkevych, Yu.A. Kolesnichenko, J.M. van Ruitenbeek

The conductance of systems containing two tunnel point-contacts and a single subsurface scatterer is investigated theoretically. The problem is solved in the approximation of s-wave scattering giving analytical expressions for the wave functions and for the conductance of the system. Conductance oscillations resulting from the interference of electron waves passing through different contacts and their interference with the waves scattered by the defect are analyzed. The prospect for determining the depth of the impurity below the metal surface by using the dependence of the conductance as a function of the distance between the contacts is discussed. It is shown that the application of an external magnetic field results in Aharonov-Bohm type oscillations in the conductance, the period of which allows detection of the depth of the defect in a double tip STM experiment.

arXiv:1011.6155v1

Spontaneous cross-section field in impurity graphene

Authors: M.B. Belonenko, N.G. Lebedev, A.V. Pak, N.N. Yanyushkina

It was found that the spontaneous electric field can occur in doped graphene in an external dc electric field. The direction of the spontaneous field is perpendicular to the applied field. This effect can be connected with the nonequilibrium electron subsystem of graphene. It was also shown up the influence of problem parameters on the characteristics of the spontaneous field.

arXiv:1011.6074v1

Quantum dynamics of a dc-SQUID coupled to an asymmetric Cooper pair transistor

Authors: A. Fay, W. Guichard, O. Buisson, F. W. J. Hekking

We present a theoretical analysis of the quantum dynamics of a superconducting circuit based on a highly asymmetric Cooper pair transistor (ACPT) in parallel to a dc-SQUID. Starting from the full Hamiltonian we show that the circuit can be modeled as a charge qubit (ACPT) coupled to an anharmonic oscillator (dc-SQUID). Depending on the anharmonicity of the SQUID, the Hamiltonian can be reduced either to one that describes two coupled qubits or to the Jaynes-Cummings Hamiltonian. Here the dc-SQUID can be viewed as a tunable micron-size resonator. The coupling term, which is a combination of a capacitive and a Josephson coupling between the two qubits, can be tuned from the very strong- to the zero-coupling regimes. It describes very precisely the tunable coupling strength measured in this circuit and explains the 'quantronium' as well as the adiabatic quantum transfer read-out.

arXiv:1011.6148v1

Theoretical investigation of the four-layered self-doped high-T$_c$ superconductors: evidence of pair tunneling effect

Authors: Tao Zhou

Based on a four-layered self-doped $t-J$ type model and the slave-boson mean-field approach, we study theoretically the superconductivity in the electron-doped and hole-doped layers. The neighbor layers are coupled through both the single electron interlayer hopping and pair tunneling effect. The superconducting gap magnitude for the electron-doped band is nearly twice of that of the hole-doped one, which contrasts to our previous understanding of the electron-hole asymmetry in high-T$_c$ superconductors but consistent with recent angle-resolved-photoemission-spectroscopy experiments in four-layered materials Ba$_2$Ca$_3$Cu$_4$O$_8$F$_2$. Our results propose that the pair tunneling effect is important to examine the multi-layered superconducting materials.

arXiv:1011.6040v1

Spin-orbit interaction and asymmetry effects on Kondo ridges at finite magnetic field

Authors: S. Grap, S. Andergassen, J. Paaske, V. Meden

We study electron transport through a serial double quantum dot with Rashba spin-orbit interaction (SOI) and Zeeman field of amplitude B in presence of local Coulomb repulsion. The linear conductance as a function of a gate voltage Vg equally shifting the levels on both dots shows two B=0 Kondo ridges which are robust against SOI as time-reversal symmetry is preserved. Resulting from the crossing of a spin-up and a spin-down level at vanishing SOI two additional Kondo plateaus appear at finite B. They are not protected by symmetry and rapidly vanish if the SOI is turned on. Left-right asymmetric level-lead couplings and detuned on-site energies lead to a simultaneous breaking of left-right and bonding-anti-bonding state symmetry. In this case the finite-B Kondo ridges in the Vg-B plane are bent with respect to the Vg-axis. For the Kondo ridge to develop different level renormalizations must be compensated by adjusting B.

arXiv:1011.5916v1

Single Photon Near Field Emission and Revival in Quantum Dots

Authors: Sergio Tafur, Michael N. Leuenberger

Models of the spontaneous emission of photons coupled to the electronic states of quantum dots are important for understanding quantum interactions in dielectric media as applied to proposed solid-state quantum computers, single photon emitters, and single photon detectors. The characteristic lifetime of photon emission is traditionally modeled in the Weisskopf-Wigner approximation. Here we model the fully quantized spontaneous emission, including near field effects, of a photon from the excited state of a quantum dot beyond theWeisskopf-Wigner approximation. We propose the use of discretized central-difference approximations to describe single photon states via single photon operators in 3+1 dimensions. We further show herein that one can shift from the traditional description of electrodynamics and quantum electrodynamics, in terms of electric and magnetic fields to one in terms of a photonic wave function and its operators using the Dirac equation for the propagation of single photons.

arXiv:1011.6566v1

Topological superconductivity and Majorana fermions in half-metal / superconductor heterostructure

Authors: Suk Bum Chung, Hai-Jun Zhang, Xiao-Liang Qi, Shou-Cheng Zhang

A half-metal is by definition spin-polarized at its Fermi level and therefore was conventionally thought to have little proximity effect with an $s$-wave superconductor. Here we show that if there is spin-orbit coupling at the interface between a single-band half-metal and an $s$-wave superconductor, $p_x +ip_y$ superconductivity would be induced on the half-metal. This can give us a topological superconductor with a single chiral Majorana edge state. We show that two atomic layers of CrO$_2$ or CrTe give us the single-band half-metal and is thus a candidate material for realizing this physics.

arXiv:1011.6422v1

Interface Landau levels in graphene monolayer-bilayer junctions

Mikito Koshino1,*, Takeshi Nakanishi2, and Tsuneya Ando1

Electronic structure of graphene monolayer-bilayer junction in a magnetic field is studied within an effective-mass approximation. The energy spectrum is characterized by the interface Landau levels, i.e., the locally flat bands appearing near the boundary region, resulting in a series of characteristic peaks in the local density of states. Their energies are independent of boundary types such as zigzag or armchair. In the atomic scale, the local density of states shows a Kekulé pattern due to the valley mixing in the armchair boundary while does not in the zigzag boundary.

DOI: 10.1103/PhysRevB.82.205436

Electroluminescence from a single nanotube–molecule–nanotube junction

Christoph W. Marquardt1,2, Sergio Grunder3, Alfred Baszczyk1,4, Simone Dehm1, Frank Hennrich1, Hilbert v. Löhneysen2,5,6, Marcel Mayor1,3,6 & Ralph Krupke1,6

The positioning of single molecules between nanoscale electrodes1, 2, 3, 4, 5, 6, 7, 8 has allowed their use as functional units in electronic devices. Although the electrical transport in such devices has been widely explored, optical measurements have been restricted to the observation of electroluminescence from nanocrystals and nanoclusters9, 10 and from molecules in a scanning tunnelling microscope setup11, 12. In this Letter, we report the observation of electroluminescence from the core of a rod-like molecule between two metallic single-walled carbon nanotube electrodes forming a rigid solid-state device. We also develop a simple model to explain the onset voltage for electroluminescence. These results suggest new characterization and functional possibilities, and demonstrate the potential of carbon nanotubes for use in molecular electronics.

doi:10.1038/nnano.2010.230

High-Performance Carbon Nanotube Light-Emitting Diodes with Asymmetric Contacts

Sheng Wang*†, Qingsheng Zeng†, Leijing Yang†, Zhiyong Zhang†, Zhenxing Wang†, Tian Pei†, Li Ding†, Xuelei Liang†, Min Gao†, Yan Li‡, and Lian-Mao Peng*†

Electroluminescence (EL) measurements are carried out on a two-terminal carbon nanotube (CNT) based light-emitting diode (LED). This two-terminal device is composed of an asymmetrically contacted semiconducting single-walled carbon nanotube (SWCNT). On the one end the SWCNT is contacted with Sc and on the other end with Pd. At large forward bias, with the Sc contact being grounded, electrons can be injected barrier-free into the conduction band of the SWCNT from the Sc contact and holes be injected into the valence band from the Pd electrode. The injected electrons and holes recombine radiatively in the SWCNT channel yielding a narrowly peaked emission peak with a full width at half-maximum of about 30 meV. Detailed EL spectroscopy measurements show that the emission is excitons dominated process, showing little overlap with that associated with the continuum states. The performance of the LED is compared with that based on a three-terminal field-effect transistor (FET) that is fabricated on the same SWCNT. The conversion efficiency of the two-terminal diode is shown to be more than three times higher than that of the FET based device, and the emission peak of the LED is much narrower and operation voltage is lower.

Nano Lett., Article ASAP DOI: 10.1021/nl101513z

Nov. 11. - nov. 25. (2010)

Válogatta: Halbritter András

Channel saturation and conductance quantization in single-atom gold constrictions

Jason N. Armstrong, R. M. Schaub, Susan Z. Hua, and Harsh Deep Chopra*

It is shown that the finite elasticity of atomic-sized gold constrictions allows for a continuous and reversible change in conductance. This is achieved by superposition of atomic-scale or subatomic-scale mechanical oscillations on a steadily retracting/approaching gold tip against a gold substrate. Through these perturbation studies, we report the direct observation of channel saturation and conductance quantization in stable, single-atom gold constrictions. In addition, the origin of peaks in conductance histograms is explained, and the peaks alone are shown to be insufficient in evaluating the stability of atomic configurations.

http://link.aps.org/doi/10.1103/PhysRevB.82.195416

Spin Aharonov-Bohm effect and topological spin transistor

Joseph Maciejko1,2, Eun-Ah Kim3, and Xiao-Liang Qi1,2

Ever since its discovery, the electron spin has only been measured or manipulated through the application of an electromagnetic force acting on the associated magnetic moment. In this work, we propose a spin Aharonov-Bohm effect in which the electron spin is controlled by a magnetic flux while no electromagnetic field is acting on the electron. Such a nonlocal spin manipulation is realized in an Aharonov-Bohm ring made from the recently discovered quantum spin Hall insulator, by taking advantage of the defining property of the quantum spin Hall edge states: the one-to-one correspondence between spin polarization and direction of propagation. The proposed setup can be used to realize a new spintronics device, the topological spin transistor, in which the spin rotation is completely controlled by a magnetic flux of hc/2e, independently of the details of the sample.

http://link.aps.org/doi/10.1103/PhysRevB.82.195409

Limits on Intrinsic Magnetism in Graphene

M. Sepioni1, R. R. Nair1, S. Rablen1, J. Narayanan1, F. Tuna2, R. Winpenny2, A. K. Geim1,†, and I. V. Grigorieva1

We have studied magnetization of graphene nanocrystals obtained by sonic exfoliation of graphite. No ferromagnetism is detected at any temperature down to 2 K. Neither do we find strong paramagnetism expected due to the massive amount of edge defects. Rather, graphene is strongly diamagnetic, similar to graphite. Our nanocrystals exhibit only a weak paramagnetic contribution noticeable below 50 K. The measurements yield a single species of defects responsible for the paramagnetism, with approximately one magnetic moment per typical graphene crystallite.

http://link.aps.org/doi/10.1103/PhysRevLett.105.207205

Is CrO2 Fully Spin Polarized? Analysis of Andreev Spectra and Excess Current

Tomas Löfwander1, Roland Grein2, and Matthias Eschrig2,3

We report an extensive theoretical analysis of point-contact Andreev reflection data available in the literature on ferromagnetic CrO2. We find that the spectra can be well understood within a model of fully spin-polarized bands in CrO2 together with spin-active scattering at the contact. This is in contrast to analysis of the data within extended Blonder-Tinkham-Klapwijk models, which lead to a spin polarization varying between 50% and 100% depending on the transparency of the interface. We propose to utilize both the temperature dependence of the spectra and the excess current at voltages above the gap to resolve the spin polarization in CrO2 in a new generation of experiments.

http://link.aps.org/doi/10.1103/PhysRevLett.105.207001

Viewing the Interior of a Single Molecule: Vibronically Resolved Photon Imaging at Submolecular Resolution

Chi Chen, Ping Chu, C. A. Bobisch, D. L. Mills, and W. Ho*

We report the spatial imaging of the photon transition probability of a single molecule at submolecular resolution. Photon imaging of a ringlike pattern is further resolved as two orthogonal vibronic transitions after incorporating spectral selectivity. A theoretical model and the calculated intensity images reveal that the transition probability is dominated by the symmetry of the positions of the tip and the transition dipole moment. This imaging technique enables the probing of the electronic and optical properties in the interior of a single molecule.

http://link.aps.org/doi/10.1103/PhysRevLett.105.217402

Tuning Energy Relaxation along Quantum Hall Channels

C. Altimiras, H. le Sueur, U. Gennser, A. Cavanna, D. Mailly, and F. Pierre

The chiral edge channels in the quantum Hall regime are considered ideal ballistic quantum channels, and have quantum information processing potentialities. Here, we demonstrate experimentally, at a filling factor of νL=2, the efficient tuning of the energy relaxation that limits quantum coherence and permits the return toward equilibrium. Energy relaxation along an edge channel is controllably enhanced by increasing its transmission toward a floating Ohmic contact, in quantitative agreement with predictions. Moreover, by forming a closed inner edge channel loop, we freeze energy exchanges in the outer channel. This result also elucidates the inelastic mechanisms at work at νL=2, informing us, in particular, that those within the outer edge channel are negligible.

http://link.aps.org/doi/10.1103/PhysRevLett.105.226804

Dynamic Hall Effect Driven by Circularly Polarized Light in a Graphene Layer

J. Karch1, P. Olbrich1, M. Schmalzbauer1, C. Zoth1, C. Brinsteiner1, M. Fehrenbacher1, U. Wurstbauer1, M. M. Glazov2, S. A. Tarasenko2, E. L. Ivchenko2, D. Weiss1, J. Eroms1, R. Yakimova3, S. Lara-Avila4, S. Kubatkin4, and S. D. Ganichev1

We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polarized terahertz radiation at room temperature generates—under oblique incidence—an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed E and B fields which are, however rotating with the light’s frequency.

http://link.aps.org/doi/10.1103/PhysRevLett.105.227402

Scaling and Interaction-Assisted Transport in Graphene with One-Dimensional Defects

M. Kindermann

We analyze the scattering from one-dimensional defects in intrinsic graphene. The Coulomb repulsion between electrons is found to be able to induce singularities of such scattering at zero temperature as in one-dimensional conductors. In striking contrast to electrons in one space dimension, however, repulsive interactions here can enhance transport. We present explicit calculations for the scattering from vector potentials that appear when strips of the material are under strain. There the predicted effects are exponentially large for strong scatterers.

http://link.aps.org/doi/10.1103/PhysRevLett.105.216602

Enhancing the Coherence of a Spin Qubit by Operating it as a Feedback Loop That Controls its Nuclear Spin Bath

Hendrik Bluhm1, Sandra Foletti1, Diana Mahalu2, Vladimir Umansky2, and Amir Yacoby1,*

In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time T2* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.

http://link.aps.org/doi/10.1103/PhysRevLett.105.216803

Válogatta: Makk Péter

The Uncertainty Principle Determines the Nonlocality of Quantum Mechanics

Jonathan Oppenheim and Stephanie Wehner

Two central concepts of quantum mechanics are Heisenberg’s uncertainty principle and a subtle form of nonlocality that Einstein famously called “spooky action at a distance.” These two fundamental features have thus far been distinct concepts. We show that they are inextricably and quantitatively linked: Quantum mechanics cannot be more nonlocal with measurements that respect the uncertainty principle. In fact, the link between uncertainty and nonlocality holds for all physical theories. More specifically, the degree of nonlocality of any theory is determined by two factors: the strength of the uncertainty principle and the strength of a property called “steering,” which determines which states can be prepared at one location given a measurement at another.

http://www.sciencemag.org/content/330/6007/1072.full

Non-Abelian operations on Majorana fermions via single charge control

Karsten Flensberg

We demonstrate that non-Abelian rotations within the degenerate groundstate manifold of a set of Majorana fermions can be realized by the addition or removal of single electrons, and propose an implementation using Coulomb blockaded quantum dots. The exchange of electrons generates rotations similar to braiding, though not in real space. Unlike braiding operations, rotations by a continuum of angles are possible, while still being partially robust against perturbations. The quantum dots can also be used for readout of the state of the Majorana system via a charge measurement.

http://arxiv.org/abs/1011.5467

Fano resonances in nanoscale structures

Andrey E. Miroshnichenko, Sergej Flach, Yuri S. Kivshar

Modern nanotechnology allows one to scale down various important devices (sensors, chips, fibers, etc.) and thus opens up new horizons for their applications. The efficiency of most of them is based on fundamental physical phenomena, such as transport of wave excitations and resonances. Short propagation distances make phase-coherent processes of waves important. Often the scattering of waves involves propagation along different paths and, as a consequence, results in interference phenomena, where constructive interference corresponds to resonant enhancement and destructive interference to resonant suppression of the transmission. Recently, a variety of experimental and theoretical work has revealed such patterns in different physical settings. The purpose of this review is to relate resonant scattering to Fano resonances, known from atomic physics. One of the main features of the Fano resonance is its asymmetric line profile. The asymmetry originates from a close coexistence of resonant transmission and resonant reflection and can be reduced to the interaction of a discrete (localized) state with a continuum of propagation modes. The basic concepts of Fano resonances are introduced, their geometrical and/or dynamical origin are explained, and theoretical and experimental studies of light propagation in photonic devices, charge transport through quantum dots, plasmon scattering in Josephson-junction networks, and matter-wave scattering in ultracold atom systems, among others are reviewed.

http://rmp.aps.org/abstract/RMP/v82/i3/p2257_1

Colloquium: Topological insulators

M. Z. Hasan, C. L. Kane

Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. The two-dimensional (2D) topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A three-dimensional (3D) topological insulator supports novel spin-polarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on HgTe∕CdTe quantum wells are described that demonstrate the existence of the edge states predicted for the quantum spin Hall insulator. Experiments on Bi1−xSbx, Bi2Se3, Bi2Te3, and Sb2Te3 are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators.

http://rmp.aps.org/abstract/RMP/v82/i4/p3045_1

Microwave Photon Counter Based on Josephson Junctions

Y.-F. Chen, D. Hover, S. Sendelbach, L. Maurer, S.T. Merkel, E.J. Pritchett, F.K. Wilhelm, R. McDermott

he strong interaction of superconducting integrated circuits with microwave photons forms the basis of circuit quantum electrodynamics (cQED), an attractive paradigm for scalable quantum computing and a test bed for quantum optics in the strong coupling regime. Most quantum optics protocols rely on photon counters to probe the state of the light field. While optical photon counters are well established, the detection of single microwave photons is exceedingly difficult due to the small energy of the photon. Here we describe a microwave photon counter based on current-biased Josephson junctions. The absorption of a single microwave photon causes a junction to transition to the voltage state, producing a large and easily measured classical signal. With a two-junction circuit, we have performed a microwave version of the Hanbury Brown and Twiss experiment and demonstrated a clear signature of microwave photon bunching for a thermal source. The design is readily scalable to tens of parallelized junctions, a configuration that would allow number-resolved counting of microwave photons.

http://arxiv.org/abs/1011.4329

Mechanical control of vibrational states in single-molecule junctions

Youngsang Kim, Hyunwook Song, Florian Strigl, Hans-Fridtjof Pernau, Takhee Lee, Elke Scheer

We report on inelastic electron tunneling spectroscopy measurements carried out on single molecules incorporated into a mechanically controllable break-junction of Au and Pt electrodes at low temperature. Here we establish a correlation between the molecular conformation and conduction properties of a single-molecule junction. We demonstrate that the conductance through single molecules crucially depends on the contact material and configuration by virtue of their mechanical and electrical properties. Our findings prove that the charge transport via single molecules can be manipulated by varying both the molecular conformation (e.g., trans or gauche) and the contact material.

http://arxiv.org/abs/1011.3226

Current and noise correlations in a double dot Cooper pair beam splitter

D. Chevallier, J. Rech, T. Jonckheere, T. Martin

We consider a double quantum dot coupled to two normal leads and one superconducting lead, modeling the Cooper pair beam splitter studied in two recent experiments. Starting from a microscopic Hamiltonian we derive a general expression for the branching current and the noise crossed correlations in terms of single and two-particle Green's function of the dot electrons. We then study numerically how these quantities depend on the energy configuration of the dots and the presence of direct tunneling between them, isolating the various processes which come into play. In absence of direct tunneling, the antisymmetric case (the two levels have opposite energies with respect to the superconducting chemical potential) optimizes the Crossed Andreev Reflection (CAR) process while the symmetric case (the two levels have the same energies) favors the Elastic Cotunneling (EC) process. Switching on the direct tunneling tends to suppress the CAR process, leading to negative noise crossed correlations over the whole voltage range for large enough direct tunneling.

http://arxiv.org/abs/1011.3408

Conductance fluctuations in metallic nanogaps made by electromigration

P. Petit, A. Anthore, M. L. Della Rocca, P. Lafarge

We report on low temperature conductance measurements of gold nanogaps fabricated by controlled electromigration. Fluctuations of the conductance due to quantum interferences and depending both on bias voltage and magnetic field are observed. By analyzing the voltage and magnetoconductance correlation functions we determine the type of electron trajectories generating the observed quantum interferences and the effective characteristic time of phase coherence in our device.

http://arxiv.org/abs/1011.5144

Visualization of charge transport through Landau levels in graphene

G. Nazin, Y. Zhang, L. Zhang, E. Sutter & P. Sutter

Band bending and the associated spatially inhomogeneous population of Landau levels play a central role in the physics of the quantum Hall effect (QHE) by constraining the pathways for charge-carrier transport and scattering1. Recent progress in understanding such effects in low-dimensional carrier gases in conventional semiconductors has been achieved by real-space mapping using local probes2, 3. Here, we use spatially resolved photocurrent measurements in the QHE regime to study the correlation between the distribution of Landau levels and the macroscopic transport characteristics in graphene. Spatial maps show that the net photocurrent is determined by hot carriers transported to the periphery of the graphene channel, where QHE edge states provide efficient pathways for their extraction to the contacts. The photocurrent is sensitive to the local filling factor, which allows us to reconstruct the local charge density in the entire conducting channel of a graphene device.

http://www.nature.com/nphys/journal/v6/n11/full/nphys1745.html

More than just room temperature

Diluted magnetic semiconductors and oxides are interesting for fundamental science and applications even without room-temperature ferromagnetism.

http://www.nature.com/nmat/journal/v9/n12/full/nmat2918.html

A ten-year perspective on dilute magnetic semiconductors and oxides

Tomasz Dietl

In 2000, a seminal study predicted ferromagnetism above room temperature in diluted magnetic semiconductors and oxides, fuelling tremendous research activity that has lasted for a decade. Tomasz Dietl reviews the progress in understanding these materials over the past ten years, with a view to the future of semiconductor spintronics.

http://www.nature.com/nmat/journal/v9/n12/full/nmat2898.html

A window on the future of spintronics

Hideo Ohno

Despite low transition temperatures, ferromagnetism in diluted magnetic semiconductors has been essential in exploring new ideas and concepts in spintronics, some of which have been successfully transferred to metallic ferromagnets.

http://www.nature.com/nmat/journal/v9/n12/full/nmat2913.html

Molecular Bridging of Silicon Nanogaps

Geoffrey J. Ashwell, Laurie J. Phillips, Benjamin J. Robinson, Barbara Urasinska-Wojcik, Colin J. Lambert, Iain M. Grace, Martin R. Bryce, Rukkiat Jitchati, Mustafa Tavasli, Timothy I. Cox, Ian C. Sage, Rachel P. Tuffin, and Shona Ray

The highly doped electrodes of a vertical silicon nanogap device have been bridged by a 5.85 nm long molecular wire, which was synthesized in situ by grafting 4-ethynylbenzaldehyde via C−Si links to the top and bottom electrodes and thereafter by coupling an amino-terminated fluorene unit to the aldehyde groups of the activated electrode surfaces. The number of bridging molecules is constrained by relying on surface roughness to match the 5.85 nm length with an electrode gap that is nominally 1 nm wider and may be controlled by varying the reaction time: the device current increases from ≤1 pA at 1 V following the initial grafting step to 10−100 nA at 1 V when reacted for 5−15 min with the amino-terminated linker and 10 μA when reacted for 16−53 h. It is the first time that both ends of a molecular wire have been directly grafted to silicon electrodes, and these molecule-induced changes are reversible. The bridges detach when the device is rinsed with dilute acid solution, which breaks the imine links of the in situ formed wire and causes the current to revert to the subpicoampere leakage value of the 4-ethynylbenzaldehyde-grafted nanogap structure.

http://pubs.acs.org/doi/abs/10.1021/nn102460z

Bringing Order to the World of Nanowire Devices by Phase Shift Lithography

Kittitat Subannajui, Firat Gder, and Margit Zacharias

Semiconductor nanowire devices have several properties which match future requirements of scaling down the size of electronics. In typical microelectronics production, a number of microstructures are aligned precisely on top of each other during the fabrication process. In the case of nanowires, this mandatory condition is still hard to achieve. A technological breakthrough is needed to accurately place nanowires at any specific position and then form devices in mass production. In this article, an upscalable process combining conventional micromachining with phase shift lithography will be demonstrated as a suitable tool for nanowire device technology. Vertical Si and ZnO nanowires are demonstrated on very large (several cm2) areas. We demonstrate how the nanowire positions can be controlled, and the resulting nanowires are used for device fabrication. As an example Si/ZnO heterojunction diode arrays are fabricated. The electrical characterization of the produced devices has also been performed to confirm the functionality of the fabricated diodes.

http://pubs.acs.org/doi/full/10.1021/nl102103w

Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets

M. Mannini, F. Pineider, C. Danieli, F. Totti, L. Sorace, Ph. Sainctavit, M.-A. Arrio, E. Otero, L. Joly, J. C. Cezar, A. Cornia & R. Sessoli

A fundamental step towards atomic- or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface1, or of a small paramagnetic molecule embedded in a nanojunction2, can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets3, 4, 5, 6 (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface7 was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices8, 9. Here we show that a preferential orientation of Fe4 complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs—their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization5, 6—can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.

http://www.nature.com/nature/journal/v468/n7322/full/nature09478.html

Atomic-scale engineering of electrodes for single-molecule contacts

Guillaume Schull, Thomas Frederiksen, Andrés Arnau, Daniel Sánchez-Portal & Richard Berndt

The transport of charge through a conducting material depends on the intrinsic ability of the material to conduct current and on the charge injection efficiency at the contacts between the conductor and the electrodes carrying current to and from the material1, 2, 3. According to theoretical considerations4, this concept remains valid down to the limit of single-molecule junctions5. Exploring this limit in experiments requires atomic-scale control of the junction geometry. Here we present a method for probing the current through a single C60 molecule while changing, one by one, the number of atoms in the electrode that are in contact with the molecule. We show quantitatively that the contact geometry has a strong influence on the conductance. We also find a crossover from a regime in which the conductance is limited by charge injection at the contact to a regime in which the conductance is limited by scattering at the molecule. Thus, the concepts of ‘good’ and ‘bad’ contacts, commonly used in macro- and mesoscopic physics, can also be applied at the molecular scale.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2010.215.html

Flexible organic transistors and circuits with extreme bending stability

Tsuyoshi Sekitani, Ute Zschieschang, Hagen Klauk & Takao Someya

Flexible electronic circuits are an essential prerequisite for the development of rollable displays, conformable sensors, biodegradable electronics and other applications with unconventional form factors. The smallest radius into which a circuit can be bent is typically several millimetres, limited by strain-induced damage to the active circuit elements. Bending-induced damage can be avoided by placing the circuit elements on rigid islands connected by stretchable wires, but the presence of rigid areas within the substrate plane limits the bending radius. Here we demonstrate organic transistors and complementary circuits that continue to operate without degradation while being folded into a radius of 100 μm. This enormous flexibility and bending stability is enabled by a very thin plastic substrate (12.5 μm), an atomically smooth planarization coating and a hybrid encapsulation stack that places the transistors in the neutral strain position. We demonstrate a potential application as a catheter with a sheet of transistors and sensors wrapped around it that enables the spatially resolved measurement of physical or chemical properties inside long, narrow tubes.

http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat2896.html

nov 3. - nov. 11. (2010)

Válogatta: Márton Attila

Giant Faraday rotation in single- and multilayer graphene

Iris Crassee, Julien Levallois, Andrew L. Walter, Markus Ostler, Aaron Bostwick, Eli Rotenberg, Thomas Seyller, Dirk van der Marel & Alexey B. Kuzmenko Nature Physics (2010) doi:10.1038/nphys1816 Received 17 May 2010 Accepted 14 September 2010 Published online 07 November 2010

The rotation of the polarization of light after passing a medium in a magnetic field, discovered by Faraday1, is an optical analogue of the Hall effect, which combines sensitivity to the carrier type with access to a broad energy range. Up to now the thinnest structures showing the Faraday rotation were several-nanometre-thick two-dimensional electron gases2. As the rotation angle is proportional to the distance travelled by the light, an intriguing issue is the scale of this effect in two-dimensional atomic crystals or films—the ultimately thin objects in condensed matter physics. Here we demonstrate that a single atomic layer of carbon—graphene—turns the polarization by several degrees in modest magnetic fields. Such a strong rotation is due to the resonances originating from the cyclotron effect in the classical regime and the inter-Landau-level transitions in the quantum regime. Combined with the possibility of ambipolar doping3, this opens pathways to use graphene in fast tunable ultrathin infrared magneto-optical devices.

http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1816.html

Graphene as a quantum surface with curvature-strain preserving dynamics

M. V. Karasev (Submitted on 10 Nov 2010)

We discuss how the curvature and the strain density of the atomic lattice generate the quantization of graphene sheets as well as the dynamics of geometric quasiparticles propagating along the constant curvature/strain levels. The internal kinetic momentum of Riemannian oriented surface (a vector field preserving the Gaussian curvature and the area) is determined.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2339v1.pdf

Tunable band gaps in bilayer graphene-BN heterostructures

Ashwin Ramasubramaniam, Doron Naveh, ELias Towe (Submitted on 10 Nov 2010)

We investigate band-gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV, and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that the graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2489v1.pdf

Energy gaps in graphene nano-constrictions with different aspect ratios

B. Terrés, J. Dauber, C. Volk, S. Trellenkamp, U. Wichmann, C. Stampfer (Submitted on 9 Nov 2010)

We present electron transport measurements on lithographically defined and etched graphene nano-constrictions with different aspect ratios, including different length (l) and width (w). A roughly length-independent effective energy gap can be observed around the charge neutrality point. This energy gap scales inversely with the width even in regimes where the length of the constriction is smaller than its width (l < w). In very short constrictions we observe in the gap region both, resonances due to localized states or charged islands and an elevated overall conductance level (0.1 - 1e2/h), which is strongly length-dependent. This makes very short graphene constrictions interesting for high transparent graphene tunneling barriers.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2091v1.pdf

Tunneling conductance in strained graphene-based superconductor: Effect of asymmetric Weyl-Dirac fermions

Bumned Soodchomshom (Submitted on 7 Nov 2010)

Based on the BTK theory, we investigate the tunneling conductance in a uniaxially strained graphene-based normal metal (NG)/ barrier (I)/superconductor (SG) junctions. In the present model, we assume that depositing the conventional superconductor on the top of the uniaxially strained graphene, normal graphene may turn to superconducting graphene with the Cooper pairs formed by the asymmetric Weyl-Dirac electrons, the massless fermions with direction-dependent velocity. The highly asymmetrical velocity, vy/vx>>1, may be created by strain in the zigzag direction near the transition point between gapless and gapped graphene. In the case of the highly asymmetrical velocity, we find that the Andreev reflection strongly depends on the direction and the current perpendicular to the direction of strain can flow in the junction as if there was no barrier. Also, the current parallel to the direction of strain anomalously oscillates as a function of the gate voltage with very high frequency. Our predicted result is found as quite different from the feature of the quasiparticle tunneling in the unstrained graphene-based NG/I/SG conventional junction. This is because of the presence of the direction-dependent-velocity quasiparticles in the highly strained graphene system.

http://xxx.lanl.gov/ftp/arxiv/papers/1011/1011.1617.pdf

Raman and optical characterization of multilayer turbostratic graphene grown via chemical vapor deposition

Daniel R. Lenski, Michael S. Fuhrer (Submitted on 7 Nov 2010 (v1), last revised 9 Nov 2010 (this version, v2))

We synthesize large-area graphene via atmospheric-pressure (AP) chemical vapor deposition (CVD) on copper, and transfer to SiO2 wafers. In contrast to low-pressure (LP) CVD on copper, optical contrast and atomic force microscopy measurements show AP-CVD graphene contains significant multi-layer areas. Raman spectroscopy always shows a single Lorentzian 2D peak, however systematic differences are observed in the 2D peak energy, width, and intensity for single- and multi-layer regions. We conclude that graphene multi-layers grown by AP-CVD on Cu are rotationally disordered.

http://xxx.lanl.gov/ftp/arxiv/papers/1011/1011.1683.pdf

Regularization of the spectral problem for the monolayer graphene with the separable in the angular momentum representation singular potential of defect

Sergey A. Ktitorov, Yurii I. Kuzmin, Natalie E. Firsova (Submitted on 4 Nov 2010)

The electronic states in the monolayer graphene with the short-range perturbation asymmetric with respect to the band index are analized. The study was made for the separable in the angular momentum representation potential basing on the (2+1)-dimensional Dirac equation. The characteristic equation for bound and resonance states obtained in the present paper is compared with one derived earlier for the same problem with different approach. The momentum representation approach used in the present paper allowed us to obtain the satisfactory regularization of the Hadamar incorrect boundary problem stemming from the potential singularity.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1231v1.pdf

Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions

Martin Amft, Sébastien Lebègue, Olle Eriksson, Natalia V. Skorodumova (Submitted on 4 Nov 2010)

We performed a systematic density functional study of the adsorption of copper, silver, and gold adatoms on graphene, especially accounting for van der Waals interactions by the vdW-DF and the PBE+D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene can be significant. The results for silver are at variance with those obtained with GGA, which gives no binding in this case. However, we observe some quantitative differences between the vdW-DF and the PBE+D2 methods. For instance the adsorption energies calculated with the PBE+D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE+D2 are shorter than those calculated with the vdW-DF method.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1113v1.pdf

Influence of electron-hole drag on conductivity of neutral and gated graphene

I. I. Boiko (Submitted on 4 Nov 2010)

Conductivity of monolayer and two-layer graphene is considered with due regard for mutual drag of band electrons and holes. Search of contribution of the drag in conductivity shows that this effect can sufficiently influence on mobility of carriers, which belong to different groups and have different drift velocities. In two-layer system the mutual drag can even change the di-rection of partial current in separate layer.

http://xxx.lanl.gov/ftp/arxiv/papers/1011/1011.1105.pdf

Compressibility of graphene

D. S. L. Abergel, E. H. Hwang, S. Das Sarma (Submitted on 3 Nov 2010)

We develop a theory for the compressibility and quantum capacitance of disordered monolayer and bilayer graphene including the full hyperbolic band structure and band gap in the latter case. We include the effects of disorder in our theory, which are of particular importance at the carrier densities near the Dirac point. We account for this disorder statistically using two different averaging procedures: first via averaging over the density of carriers directly, and then via averaging in the density of states to produce an effective density of carriers. We also compare the results of these two models with experimental data, and to do this we introduce a model for inter-layer screening which predicts the size of the band gap between the low-energy conduction and valence bands for arbitary gate potentials applied to both layers of bilayer graphene. We find that both models for disorder give qualitatively correct results for gapless systems, but when there is a band gap at charge neutrality, the density of states averaging is incorrect and disagrees with the experimental data.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.0995v1.pdf

Surface-Enhanced Raman Signal for Terbium Single-Molecule Magnets Grafted on Graphene

Manuel Lopes†, Andrea Candini†‡, Matias Urdampilleta†, Antoine Reserbat-Plantey†, Valerio Bellini‡, Svetlana Klyatskaya§, Laetitia Marty†, Mario Ruben,, Marco Affronte, Wolfgang Wernsdorfer†, and Nedjma Bendiab†* ACS Nano, Article ASAP, Publication Date (Web): November 10, 2010

We report the preparation and characterization of monolayer graphene decorated with functionalized single-molecule magnets (SMMs). The grafting ligands provide a homogeneous and selective deposition on graphene. The grafting is characterized by combined Raman microspectroscopy, atomic force microscopy (AFM), and electron transport measurements. We observe a surface-enhanced Raman signal that allowed us to study the grafting down to the limit of a few isolated molecules. The weak interaction through charge transfer is in agreement with ab initio DFT calculations. Our results indicate that both molecules and graphene are essentially intact and the interaction is driven by van der Waals forces.

http://pubs.acs.org/doi/abs/10.1021/nn1018363

Graphene-Based Supercapacitor with an Ultrahigh Energy Density

Chenguang Liu†§, Zhenning Yu‡§, David Neff†, Aruna Zhamu‡, and Bor Z. Jang*† Nano Lett., Article ASAP DOI: 10.1021/nl102661q Publication Date (Web): November 8, 2010

A supercapacitor with graphene-based electrodes was found to exhibit a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current density of 1 A/g. These energy density values are comparable to that of the Ni metal hydride battery, but the supercapacitor can be charged or discharged in seconds or minutes. The key to success was the ability to make full utilization of the highest intrinsic surface capacitance and specific surface area of single-layer graphene by preparing curved graphene sheets that will not restack face-to-face. The curved morphology enables the formation of mesopores accessible to and wettable by environmentally benign ionic liquids capable of operating at a voltage >4 V.

http://pubs.acs.org/doi/abs/10.1021/nl102661q

The rise and rise of graphene

Nature Nanotechnology 5, 755 (2010) Published online: 4 November 2010 | doi:10.1038/nnano.2010.224

This year's Nobel Prize in Physics can be seen as part of the larger story of hexagonally bonded carbon. The recent award of the Nobel Prize in Physics to Andrei Geim and Konstantin Novoselov, two Russian-born physicists working at Manchester University in the United Kingdom, “for groundbreaking experiments regarding the two-dimensional material graphene”, is notable not just because it comes only six years after they and six others published their breakthrough paper in Science1, or because Novoselov is among the prize's youngest-ever winners (he is just 36). It is also notable because, together with the 1996 Nobel Prize in Chemistry (awarded for the discovery of fullerenes) and the 2008 Kavli Prize in Nanoscience (for carbon nanotubes), it completes a trifecta of mega-awards to three different topologies of what is otherwise exactly the same substance: hexagonally bonded carbon.

http://www.nature.com/nnano/journal/v5/n11/full/nnano.2010.224.html

Giant Stark effect in the emission of single semiconductor quantum dots

Anthony. J. Bennett, Raj. B. Patel, Joanna Skiba-Szymanska, Christine A. Nicoll, Ian Farrer, David A. Ritchie, Andrew J. Shields, (Submitted on 10 Nov 2010)

We study the quantum-confined Stark effect in single InAs/GaAs quantum dots embedded within a AlGaAs/GaAs/AlGaAs quantum well. By significantly increasing the barrier height we can observe emission from a dot at electric fields of -500 kV/cm, leading to Stark shifts of up to 25 meV. Our results suggest this technique may enable future applications that require self-assembled dots with transitions at the same energy.

http://xxx.lanl.gov/ftp/arxiv/papers/1011/1011.2436.pdf

Time-convolutionless master equation for quantum dots: Perturbative expansion to arbitrary order

Carsten Timm (Submitted on 10 Nov 2010)

The master equation describing the non-equilibrium dynamics of a quantum dot coupled to metallic leads is considered. Employing a superoperator approach, we derive an exact time-convolutionless master equation for the probabilities of dot states, i.e., a time-convolutionless Pauli master equation. The generator of this master equation is derived order by order in the hybridization between dot and leads. Although the generator turns out to be closely related to the T-matrix expressions for the transition rates, which are plagued by divergences, in the time-convolutionless generator all divergences cancel order by order. The time-convolutionless and T-matrix master equations are contrasted to the Nakajima-Zwanzig version. The absence of divergences in the Nakajima-Zwanzig master equation due to the nonexistence of secular reducible contributions becomes rather transparent in our approach, which explicitly projects out these contributions. We also show that the time-convolutionless generator contains the generator of the Nakajima-Zwanzig master equation in the Markov approximation plus corrections, which we make explicit. Furthermore, it is shown that the stationary solutions of the time-convolutionless and the Nakajima-Zwanzig master equations are identical. However, this identity neither extends to perturbative expansions truncated at finite order nor to dynamical solutions. We discuss the conditions under which the Nakajima-Zwanzig-Markov master equation nevertheless yields good results.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2371v1.pdf

Quantum coherence versus quantum discord in two coupled semiconductor double-dot molecules via a transmission line resonator

Pei Pei, Chong Li, Jia-Sen Jin, He-Shan Song (Submitted on 10 Nov 2010)

We study the dynamics of quantum coherence and quantum correlations in two semiconductor double-dot molecules separated by a distance and indirectly coupled via a transmission line resonator. Dominant dissipation processes are considered. The numerical results show the sudden death of entanglement and the robustness of quantum discord to sudden death. Furthermore, the results indicate the dephasing processes in our model can lead in the revival and decay of coherence and discord with the absence of entanglement for certain initial states. By observing the dynamics of coherence versus discord for different initial states, we find that the similarities and differences of coherence and discord are not only related to the dependance of discord on optimizing the measurement set, but more importantly to the coherences in individual qubits which are captured by the adopted coherence measure.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2252v1.pdf

Manipulation of the Lande g-factor in InAs quantum dots through application of anisotropic gate potentials

Sanjay Prabhakar, James E Raynolds, Roderick Melnik (Submitted on 8 Nov 2010)

We study the variation in the Lande g-factor of electron spins induced by an anisotropic gate potential in InAs quantum dots for potential use as non-charge based logic devices. In this paper, we present the numerical simulations of such spins in an electrostatically confined two-dimensional asymmetric gate potential forming a quantum dot system in a 2DEG. Using numerical techniques, we show that the broken in-plane rotational symmetry, due to Rashba spin orbit coupling in an asymmetric potential (induced by gate voltages) leads to a significant reverse effect on the tunability of the electron g-factor over a symmetric model potential (i.e. the derivative of the g-factor with respect to electric field has opposite sign in the two cases).

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1921v1.pdf

Spin-orbit coupling and phase-coherence in InAs nanowires

S. Estévez Hernández, M. Akabori, K. Sladek, Ch. Volk, S. Alagha, H. Hardtdegen, N. Demarina, D. Grützmacher, Th. Schäpers, M. G. Pala (Submitted on 6 Nov 2010)

We investigated the magnetotransport of InAs nanowires grown by selective area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation of the magnetic field or the back-gate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length l_phi is determined. At the lowest temperatures l_phi is found to be at least 300 nm, while for temperatures exceeding 2 K a monotonous decrease of l_phi with temperature is observed. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctuations. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the experimental data to simulations based on a recursive two-dimensional Green's function approach a spin-orbit scattering length of approximately 70 nm was extracted, indicating the presence of strong spin-orbit coupling.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1556v1.pdf

Interactions of nanorod particles in the strong coupling regime

Cheng-ping Huang, Xiao-gang Yin, Ling-bao Kong, Yong-yuan Zhu (Submitted on 5 Nov 2010)

The plasmon coupling in a nanorod dimer obeys the exponential size dependence according to the Universal Plasmon Ruler Equation. However, it was shown recently that such a model does not hold at short nanorod distance (Nano Lett. 2009, 9, 1651). Here we study the nanorod coupling in various cases, including nanorod dimer with the asymmetrical lengths and symmetrical dimer with the varying gap width. The asymmetrical nanorod dimer causes two plasmon modes: one is the attractive lower- energy mode and the other the repulsive high-energy mode. Using a simple coupled LC-resonator model, the position of dimer resonance has been determined analytically. Moreover, we found that the plasmon coupling of symmetrical cylindrical (or rectangular) nanorod dimer is governed uniquely by gap width scaled for the (effective) rod radius rather than for the rod length. A new Plasmon Ruler Equation without using the fitting parameters has been proposed, which agrees well with the FDTD calculations. The method has also been extended to study the plasmonic wave-guiding in a linear chain of gold nanorod particles. A field decay length up to 2700nm with the lateral mode size about 50nm (~wavelength/28) has been suggested.

http://xxx.lanl.gov/ftp/arxiv/papers/1011/1011.1408.pdf

Single-shot initialization of electron spin in a quantum dot using a short optical pulse

Vivien Loo, Loic Lanco, Olivier Krebs, Pascale Senellart, Paul Voisin (Submitted on 4 Nov 2010)

We propose a technique to initialize an electron spin in a semiconductor quantum dot with a single short optical pulse. It relies on the fast depletion of the initial spin state followed by a preferential, Purcell-accelerated desexcitation towards the desired state thanks to a micropillar cavity. We theoretically discuss the limits on initialization rate and fidelity, and derive the pulse area for optimal initialization. We show that spin initialization is possible using a single optical pulse down to a few tens of picoseconds wide.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1156v1.pdf

Quantum dot-cavity strong-coupling regime measured through coherent reflection spectroscopy in a very high-Q micropillar

Vivien Loo, Loic Lanco, Aristide Lemaitre, Isabelle Sagnes, Olivier Krebs, Paul Voisin, Pascale Senellart (Submitted on 4 Nov 2010)

We report on the coherent reflection spectroscopy of a high-quality factor micropillar, in the strong coupling regime with a single InGaAs annealed quantum dot. The absolute reflectivity measurement is used to study the characteristics of our device at low and high excitation power. The strong coupling is obtained with a g=16 \mueV coupling strength in a 7.3\mum diameter micropillar, with a cavity spectral width kappa=20.5 \mueV (Q=65 000). The factor of merit of the strong-coupling regime, 4g/kappa=3, is the current state-of-the-art for a quantum dot-micropillar system.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1155v1.pdf

The Kondo crossover in shot noise of a single quantum dot with orbital degeneracy

Rui Sakano, Tatsuya Fujii, Akira Oguri (Submitted on 10 Nov 2010)

We investigate out of equilibrium transport through an orbital Kondo system realized in a single quantum dot, described by the multiorbital impurity Anderson model. Shot noise and current are calculated up to the third order in bias voltage in the particle-hole symmetric case, using the renormalized perturbation theory. The derived expressions are asymptotically exact at low energies. The resulting Fano factor of the backscattering current $F_b$ is expressed in terms of the Wilson ratio $R$ and the orbital degeneracy $N$ as $F_b =\frac{1 + 9(N-1)(R-1)^2}{1 + 5(N-1)(R-1)^2}$ at zero temperature. Then, for small Coulomb repulsions $U$, we calculate the Fano factor exactly up to terms of order $U^5$, and also carry out the numerical renormalization group calculation for intermediate $U$ in the case of two- and four-fold degeneracy ($N=2,\,4$). As $U$ increases, the charge fluctuation in the dot is suppressed, and the Fano factor varies rapidly from the noninteracting value $F_b=1$ to the value in the Kondo limit $F_b=\frac{N+8}{N+4}$, near the crossover region $U\sim \pi \Gamma$, with the energy scale of the hybridization $\Gamma$.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2343v1.pdf

Detection of spin injection into a double quantum dot: Violation of magnetic-field-inversion symmetry of nuclear polarization instabilities

Mark S. Rudner, Emmanuel I. Rashba (Submitted on 10 Nov 2010)

In mesoscopic systems with spin-orbit coupling, spin-injection into quantum dots at zero magnetic field is expected under a wide range of conditions. However, up to now, a viable approach for experimentally identifying such injection has been lacking. We show that electron spin injection into a spin-blockaded double quantum dot is dramatically manifested in the breaking of magnetic- field-inversion symmetry of nuclear polarization instabilities. Over a wide range of parameters, the asymmetry between positive and negative instability fields is extremely sensitive to the injected electron spin polarization and allows for the detection of even very weak spin injection. This phenomenon may be used to investigate the mechanisms of spin transport, and may hold implications for spin-based information processing.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2247v1.pdf

Tomography of the two-electron spin qubits in semiconductors

Zhan Su, Tao Tu, Gang Cao, Guang-Can Guo, Guo-Ping Guo (Submitted on 9 Nov 2010 (v1), last revised 10 Nov 2010 (this version, v2))

We propose an approach to reconstruct two-electron spin qubit states in semiconductor quantum dots by employing tomographic techniques. This procedure exploits the combination of fast gate operations on electron spins trapped in dots and dynamical nuclear polarization of the underlying Ga and As nuclei. The presented method can be an important tool in solid state quantum computation for complete characterization of qubit states and theirs correlations. link?

CdSe Quantum Dots for Two-Photon Fluorescence Thermal Imaging

Laura Martinez Maestro†, Emma Mart n Rodríguez‡, M. C. Iglesias-de la Cruz‡, Angeles Juarranz‡, Rafik Naccache§, Fiorenzo Vetrone, Daniel Jaque†, John A. Capobianco*, and Jose García Solé Nano Lett., Article ASAP DOI: 10.1021/nl1036098 Publication Date (Web): November 9, 2010

The technological development of quantum dots has ushered in a new era in fluorescence bioimaging, which was propelled with the advent of novel multiphoton fluorescence microscopes. Here, the potential use of CdSe quantum dots has been evaluated as fluorescent nanothermometers for two-photon fluorescence microscopy. In addition to the enhancement in spatial resolution inherent to any multiphoton excitation processes, two-photon (near-infrared) excitation leads to a temperature sensitivity of the emission intensity much higher than that achieved under one-photon (visible) excitation. The peak emission wavelength is also temperature sensitive, providing an additional approach for thermal imaging, which is particularly interesting for systems where nanoparticles are not homogeneously dispersed. On the basis of these superior thermal sensitivity properties of the two-photon excited fluorescence, we have demonstrated the ability of CdSe quantum dots to image a temperature gradient artificially created in a biocompatible fluid (phosphate-buffered saline) and also their ability to measure an intracellular temperature increase externally induced in a single living cell.

http://pubs.acs.org/doi/abs/10.1021/nl1036098

Effect of Dephasing on Electron Transport in a Molecular Wire: Green's Function Approach

Moumita Dey, Santanu K. Maiti, S. N. karmakar (Submitted on 9 Nov 2010)

The effect of dephasing on electron transport through a benzene molecule is carefully examined using a phenomenological model introduced by B\"{u}ttiker. Within a tight-binding framework all the calculations are performed based on the Green's function formalism. We investigate the influence of dephasing on transmission probability and current-voltage characteristics for three different configurations ({\em ortho}, {\em meta} and {\em para}) of the molecular system depending on the locations of two contacting leads. The presence of dephasing provides a significant change in the spectral properties of the molecule and exhibits several interesting patterns that have so far remain unexplored.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2033v1.pdf

Electrical transport through a mechanically gated molecular wire

C. Toher, R. Temirov, A. Greuling, F. Pump, M. Kaczmarski, M. Rohlfing, G. Cuniberti, F.S. Tautz (Submitted on 5 Nov 2010)

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.

http://xxx.lanl.gov/ftp/arxiv/papers/1011/1011.1400.pdf

Self consistent charge-current in a mesoscopic region attached to superconductor leads

D Verrilli, F Marin (Submitted on 5 Nov 2010)

We investigate the behavior of a electric potential profile inside a mesoscopic region attached to a pair of superconducting leads. It turns out that ${\rm I}-V$ characteristic curves are strongly modified by this profile. In addition, the electronic population in the mesoscopic region is affected by the profile behavior. We discuss the single particle current and the mesoscopic electronic population within the non-equilibrium Keldysh Green functions technique. The Keldysh technique results are further converted in a self consistent field (SFC) problem by introducing potential profile modifications as proposed by Datta. Evaluation of ${\rm I}-V$ characteristics are presented for several values of the model parameters and comparison with current experimental results are discussed.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.1372v1.pdf

Spin Soret effect

Sylvain D. Brechet, Jean-Philippe Ansermet (Submitted on 10 Nov 2010)

Using a three-current model for heat, spin-up and spin-down electrons, the thermodynamics of irreversible processes predicts that a temperature gradient gives rise to a spin current under the conditions used to measure what is called the spin Seebeck effect. A diffusive current proportional to a gradient of the chemical potential is known in thermochemistry as a Soret effect or thermophoresis.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2323v1.pdf

An electron jet pump: The Venturi effect of a Fermi liquid

D. Taubert, G. J. Schinner, C. Tomaras, H. P. Tranitz, W. Wegscheider, S. Ludwig (Submitted on 10 Nov 2010)

A three-terminal device based on a two-dimensional electron system is investigated in the regime of non-equilibrium transport. Excited electrons scatter with the cold Fermi sea and transfer energy and momentum to other electrons. A geometry analogous to a water jet pump is used to create a jet pump for electrons. Because of its phenomenological similarity we name the observed behavior "electronic Venturi effect".

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2289v1.pdf

Andreev spectroscopy and surface density of states for a three-dimensional time-reversal invariant topological superconductor

Andreas P. Schnyder, P. M. R. Brydon, Dirk Manske, Carsten Timm (Submitted on 23 Aug 2010 (v1), last revised 10 Nov 2010 (this version, v2))

A topological superconductor is a fully gapped superconductor that exhibits exotic zero-energy Andreev surface states at interfaces with a normal metal. In this paper we investigate the properties of a three-dimensional time reversal invariant topological superconductor by means of a two-band model with unconventional pairing in both the inter- and intraband channels. Due to the bulk-boundary correspondence the presence of Andreev surface states in this system is directly related to the topological structure of the bulk wavefunctions, which is characterized by a winding number. Using quasiclassical scattering theory we construct the spectrum of the Andreev bound states that appear near the surface and compute the surface density of states for various surface orientations. Furthermore, we consider the effects of band splitting, i.e., the breaking of an inversion-type symmetry, and demonstrate that in the absence of band splitting there is a direct transition between the fully gapped topologically trivial phase and the nontrivial phase, whereas in the presence of band splitting there exists a finite region of a gapless nodal superconducting phase between the fully gapped topologically trivial and nontrivial phases.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1008/1008.3762v2.pdf

Observation of exchange Coulomb interactions in the quantum Hall state at nu=3

A.B.Van'kov, T.D.Rhone, A.Pinczuk, I.V.Kukushkin, L.N.Pfeiffer, K.W.West (Submitted on 9 Nov 2010)

Coulomb exchange interactions of electrons in the nu=3 quantum Hall state are determined from two inter-Landau level spin-flip excitations measured by resonant inelastic light scattering. The two coupled collective excitations are linked to inter-Landau level spin-flip transitions arising from the N=0 and N=1 Landau levels. The strong repulsion between the two spin-flip modes in the long-wave limit is clearly manifested in spectra displaying Coulomb exchange contributions that are comparable to the exchange energy for the quantum Hall state at nu=1. Theoretical calculations within the Hartree-Fock approximation are in a good agreement with measured energies of spin-flip collective excitations.

http://xxx.lanl.gov/PS_cache/arxiv/pdf/1011/1011.2022v1.pdf

Electron microscopy: A new spin on electron beams

Huolin L. Xin1 & David A. Muller2 Nature Nanotechnology 5, 764 - 765 (2010) Published online: 4 November 2010 | doi:10.1038/nnano.2010.222

Ideas about angular momentum that have been borrowed from optics could allow the magnetic and spin structures of materials to be studied on atomic scales with electron vortex beams. We are used to electrons occupying states with quantized spin and orbital angular momenta in atoms, but existing methods for generating high-energy electron beams are very inefficient at either producing or transferring states with well-defined spin and/or orbital angular momentum. This failure stems from the dominance of the direct Coulomb interaction over the spin–spin and spin–orbit interactions for high-energy electrons.

http://www.nature.com/nnano/journal/v5/n11/full/nnano.2010.222.html

Biomolecular computing: Learning through play

Vladimir Privman1 Nature Nanotechnology 5, 767 - 768 (2010) Published online: 4 November 2010 | doi:10.1038/nnano.2010.221

Solutions of DNA-based molecules can be taught to play a simple game in a process that does not require the operator to be familiar with the underlying molecular programming. As the boundaries of silicon electronics have been pushed to smaller and smaller feature sizes, a growing interest in unconventional computing has emerged. A number of alternative approaches have been considered, including quantum computing, which has grown into a major research field, but many of these rely on nanoscale features and, as such, they share the scaling-down challenges present in silicon electronics.

http://www.nature.com/nnano/journal/v5/n11/full/nnano.2010.221.html

Okt. 21. - nov. 3. (2010)

Válogatta: Csonka Szabolcs

Spin-orbit qubit in a semiconductor nanowire

S. Nadj-Perge, S.M. Frolov, E.P.A.M. Bakkers, L.P. Kouwenhoven

Motion of electrons can influence their spins through a fundamental effect called spin-orbit interaction. This interaction provides a way to electrically control spins and as such lies at the foundation of spintronics. Even at the level of single electrons, spin-orbit interaction has proven promising for coherent spin rotations. Here we report a spin-orbit quantum bit implemented in an InAs nanowire, where spin-orbit interaction is so strong that spin and motion can no longer be separated. In this regime we realize fast qubit rotations and universal single qubit control using only electric fields. We enhance coherence by dynamically decoupling the qubit from the environment. Our qubits are individually addressable: they are hosted in single-electron quantum dots, each of which has a different Land\'e g-factor. The demonstration of a nanowire qubit opens ways to harness the advantages of nanowires for use in quantum computing. Nanowires can serve as one-dimensional templates for scalable qubit registers. Unique to nanowires is the possibility to easily vary the material even during wire growth. Such flexibility can be used to design wires with suppressed decoherence and push semiconductor qubit fidelities towards error-correction levels. Furthermore, electrical dots can be integrated with optical dots in p-n junction nanowires. The coherence times achieved here are sufficient for the conversion of an electronic qubit into a photon, the flying qubit, for long-distance quantum communication.

arXiv:1011.0064

Cooling electrons from 1 K to 400 mK with V-based nanorefrigerators

Authors: O. Quaranta, P. Spathis, F. Beltram, F. Giazotto (Submitted on 2 Nov 2010)

The fabrication and operation of V-based superconducting nanorefrigerators is reported. Specifically, electrons in an Al island are cooled thanks to hot-quasiparticle extraction provided by tunnel-coupled V electrodes. Electronic temperature reduction down to 400 mK starting from 1 K is demonstrated with a cooling power ~20 pW at 1 K for a junction area of 0.3 micron^2. The present architecture extends to higher temperatures refrigeration based on tunneling between superconductors and paves the way to the implementation of a multi-stage on-chip cooling scheme operating from above 1 K down to the mK regime.

arXiv:1011.0588v1

Effect of oxygen plasma etching on graphene studied with Raman spectroscopy and electronic transport

Authors: Isaac Childres, Luis A. Jauregui, Jifa Tian, Yong P. Chen (Submitted on 2 Nov 2010)

Abstract: We report a study of graphene and graphene field effect devices after exposure to a series of short pulses of oxygen plasma. We present data from Raman spectroscopy, back-gated field-effect and magneto-transport measurements. The intensity ratio between Raman "D" and "G" peaks, I(D)/I(G) (commonly used to characterize disorder in graphene) is observed to increase approximately linearly with the number (N(e)) of plasma etching pulses initially, but then decreases at higher Ne. We also discuss implications of our data for extracting graphene crystalline domain sizes from I(D)/I(G). At the highest Ne measured, the "2D" peak is found to be nearly suppressed while the "D" peak is still prominent. Electronic transport measurements in plasma-etched graphene show an up-shifting of the Dirac point, indicating hole doping. We also characterize mobility, quantum Hall states, weak localization and various scattering lengths in a moderately etched sample. Our findings are valuable for understanding the effects of plasma etching on graphene and the physics of disordered graphene through artificially generated defects.

arXiv:1011.0484v1

Mesoscopic admittance of a double quantum dot

Authors: Audrey Cottet, Christophe Mora, Takis Kontos (Submitted on 1 Nov 2010)

Abstract: We calculate the mesoscopic admittance $G(\omega)$ of a double quantum dot, which can be measured directly using microwave techniques. This quantity shows a rich behavior. In particular, it is directly sensitive to Pauli spin blockade. We then discuss the problem of DQD coupled to a high quality photonic resonator. When the photon correlation functions can be developed along a RPA-like scheme, the behavior of the resonator can be predicted from $G(\omega)$

arXiv:1011.0386v1

Edge stability, reconstruction, zero-energy states and magnetism in triangular graphene quantum dots with zigzag edges

Authors: Oleksandr Voznyy, Alev Devrim Güçlü, Pawel Potasz, Pawel Hawrylak (Submitted on 1 Nov 2010)

Abstract: We present the results of ab-initio density functional theory based calculations of the stability and reconstruction of zigzag edges in triangular graphene quantum dots. We show that, while the reconstructed pentagon-heptagon zigzag edge structure is more stable in the absence of hydrogen, ideal zigzag edges are energetically favored by hydrogen passivation. Zero-energy band exists in both structures when passivated by hydrogen, however in case of pentagon-heptagon zigzag, this band is found to have stronger dispersion, leading to the loss of net magnetization.

arXiv:1011.0369v1

Interplay of the Kondo Effect and Spin-Polarized Transport in Magnetic Molecules, Adatoms and Quantum Dots

Authors: Maciej Misiorny, Ireneusz Weymann, Jozef Barnas (Submitted on 28 Oct 2010)

Abstract: We study the interplay of the Kondo effect and spin-polarized tunneling in a class of systems exhibiting uniaxial magnetic anisotropy, such as magnetic molecules, magnetic adatoms, or quantum dots coupled to a single localized magnetic moment. Using the numerical renormalization group method we calculate the spectral functions and linear conductance in the Kondo regime. We show that the exchange coupling between conducting electrons and localized magnetic core generally leads to suppression of the Kondo effect. We also predict a nontrivial dependence of the tunnel magnetoresistance on the strength of exchange coupling and on the anisotropy constant.

arXiv:1010.6030v1

Quantum dot spin filter in Kondo regime

Authors: Mikio Eto, Tomohiro Yokoyama (Submitted on 28 Oct 2010)

Abstract: A quantum dot with spin-orbit interaction can work as an efficient spin filter if it is connected to N (> 2) external leads via tunnel barriers. When an unpolarized current is injected to the quantum dot from a lead, polarized currents are ejected to the other leads. A two-level quantum dot is examined as a minimal model. First, we show that the spin polarization is remarkably enhanced by the resonant tunneling when the level spacing in the dot is smaller than the level broadening. Next, we examine the many-body resonance by the Kondo effect in the Coulomb blockade regime. A large spin current is generated in the presence of SU(4) Kondo effect when the level spacing is smaller than the Kondo temperature.

arXiv:1010.5956v1

π junction transition in InAs self-assembled quantum dot coupled with SQUID

Authors: S. Kim, R. Ishiguro, M. Kamio, Y. Doda, E. Watanebe, K. Shibata, K. Hirakawa, H. Takayanagi (Submitted on 28 Oct 2010)

Abstract: We report the transport measurements on the InAs self-assembled quantum dots (SAQDs) which have a unique structural zero-dimensionality, coupled to a superconducting quantum interference device (SQUID). Owing to the SQUID geometry, we directly observe a {\pi} phase shift in the current phase relation and the negative supercurrent indicating {\pi} junction behavior by not only tuning the energy level of SAQD by back-gate but also controlling the coupling between SAQD and electrodes by side-gate. Our results inspire new future quantum information devices which can link optical, spin, and superconducting state.

arXiv:1010.5892v1

Tunable Non-local Coupling between Kondo Impurities

Authors: Daniel Tutuc, Bogdan Popescu, Dieter Schuh, Werner Wegscheider, Rolf J. Haug (Submitted on 27 Oct 2010)

Abstract: We study the tuning mechanisms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between two lateral quantum dots in the Kondo regime. At zero magnetic field we observe the expected splitting of the Kondo resonance and measure the RKKY interaction strength J as a function of the asymmetry between the two Kondo temperatures. At finite magnetic fields a chiral coupling between the quantum dots is observed in the Kondo chessboard and we probe the presence of the exchange interaction by analyzing the Kondo temperature with magnetic field.

arXiv:1010.5692v1

Coupling molecular spin states by photon-assisted tunneling

Authors: L. R. Schreiber, F. R. Braakman, T. Meunier, V. Calado, J. Danon, J. M. Taylor, W. Wegscheider, L. M. K. Vandersypen (Submitted on 27 Oct 2010)

Abstract: Artificial molecules containing just one or two electrons provide a powerful platform for studies of orbital and spin quantum dynamics in nanoscale devices. A well-known example of these dynamics is tunneling of electrons between two coupled quantum dots triggered by microwave irradiation. So far, these tunneling processes have been treated as electric dipole-allowed spin-conserving events. Here we report that microwaves can also excite tunneling transitions between states with different spin. In this work, the dominant mechanism responsible for violation of spin conservation is the spin-orbit interaction. These transitions make it possible to perform detailed microwave spectroscopy of the molecular spin states of an artificial hydrogen molecule and open up the possibility of realizing full quantum control of a two spin system via microwave excitation.

arXiv:1010.5682v1

Spatially probed electron-electron scattering in a two-dimensional electron gas

M. P. Jura, M. Grobis, M. A. Topinka, L. N. Pfeiffer, K. W. West, D. Goldhaber-Gordon

Using scanning gate microscopy (SGM), we probe the scattering between a beam of electrons and a two-dimensional electron gas (2DEG) as a function of the beam's injection energy, and distance from the injection point. At low injection energies, we find electrons in the beam scatter by small-angles, as has been previously observed. At high injection energies, we find a surprising result: placing the SGM tip where it back-scatters electrons increases the differential conductance through the system. This effect is explained by a non-equilibrium distribution of electrons in a localized region of 2DEG near the injection point. Our data indicate that the spatial extent of this highly non-equilibrium distribution is within ~1 micrometer of the injection point. We approximate the non-equilibrium region as having an effective temperature that depends linearly upon injection energy.

arXiv:1011.0106v1

Cooling electrons from 1 K to 400 mK with V-based nanorefrigerators

O. Quaranta, P. Spathis, F. Beltram, F. Giazotto

The fabrication and operation of V-based superconducting nanorefrigerators is reported. Specifically, electrons in an Al island are cooled thanks to hot-quasiparticle extraction provided by tunnel-coupled V electrodes. Electronic temperature reduction down to 400 mK starting from 1 K is demonstrated with a cooling power ~20 pW at 1 K for a junction area of 0.3 micron^2. The present architecture extends to higher temperatures refrigeration based on tunneling between superconductors and paves the way to the implementation of a multi-stage on-chip cooling scheme operating from above 1 K down to the mK regime.

arXiv:1011.0588v1

Válogatta: Halbritter András

Large-Diameter Graphene Nanotubes Synthesized Using Ni Nanowire Templates

Rui Wang†, Yufeng Hao†, Ziqian Wang†, Hao Gong‡, and John T. L. Thong

We report a method to synthesize tubular graphene structures by chemical vapor deposition (CVD) on Ni nanowire templates, using ethylene as a precursor at growth temperature of around 750 °C. Unlike carbon nanotubes that are synthesized via conventional routes, the number of graphene layers is determined by the growth time and is independent of the tube diameter and tube length, which follow those of the nanowire template. This allows us to realize large-diameter tubes with shells comprising a few or many layers of graphene as desired. Thin graphene layers are observed to be highly crystalline, and of uniform thickness throughout the length of the nanowire. Raman analysis shows the presence of a small level of defects typical of CVD-grown graphene. The metallic core could be removed by chemical etching to result in a collapsed tube. Backgated field-effect transistor measurements were conducted on the collapsed graphene tube. This approach to the realization of tubular graphene offers new opportunities for graphene-based nanodevices.

Nano Letter ASAP October 28, 2010

Graphene Islands on Cu Foils: The Interplay between Shape, Orientation, and Defects

Joseph M. Wofford†‡, Shu Nie§, Kevin F. McCarty§, Norman C. Bartelt§, and Oscar D. Dubon

We have observed the growth of monolayer graphene on Cu foils using low-energy electron microscopy. On the (100)-textured surface of the foils, four-lobed, 4-fold-symmetric islands nucleate and grow. The graphene in each of the four lobes has a different crystallographic alignment with respect to the underlying Cu substrate. These “polycrystalline” islands arise from complex heterogeneous nucleation events at surface imperfections. The shape evolution of the lobes is well explained by an angularly dependent growth velocity. Well-ordered graphene forms only above 790 °C. Sublimation-induced motion of Cu steps during growth at this temperature creates a rough surface, where large Cu mounds form under the graphene islands. Strategies for improving the quality of monolayer graphene grown on Cu foils must address these fundamental defect-generating processes.

Nano Letters ASAP October 27, 2010

Controllable N-Doping of Graphene

Beidou Guo†‡, Qian Liu†, Erdan Chen†, Hewei Zhu†, Liang Fang‡, and Jian Ru Gong*†

Opening and tuning an energy gap in graphene are central to many electronic applications of graphene. Here we report N-doped graphene obtained by NH3 annealing after N+-ion irradiation of graphene samples. First, the evolution of the graphene microstructure was investigated following N+-ion irradiation at different fluences using Raman spectroscopy, showing that defects were introduced in plane after irradiation and then restored after annealing in N2 or in NH3. Auger electron spectroscopy (AES) of the graphene annealed in NH3 after irradiation showed N signal, however, no N signal was observed after annealing in N2. Last, the field-effect transistor (FET) was fabricated using N-doped graphene and monitored by the source−drain conductance and back-gate voltage (Gsd−Vg) curves in the measurement. The transport property changed compared to that of the FET made by intrinsic graphene, that is, the Dirac point position moved from positive Vg to negative Vg, indicating the transition of graphene from p-type to n-type after annealing in NH3. Our approach, which provides a physical mechanism for the introduction of defect and subsequent hetero dopant atoms into the graphene material in a controllable fashion, will be promising for producing graphene-based devices for multiple applications.

Nano Letters ASAP October 22, 2010

Surface State Transport and Ambipolar Electric Field Effect in Bi2Se3 Nanodevices

Hadar Steinberg, Dillon R. Gardner, Young S. Lee, and Pablo Jarillo-Herrero

Electronic transport experiments involving the topologically protected states found at the surface of Bi2Se3 and other topological insulators require fine control over carrier density, which is challenging with existing bulk-doped material. Here we report on electronic transport measurements on thin (<100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel surface and bulk contributions. Moreover, the conductance dependence on top-gate voltage is ambipolar, consistent with tuning between electrons and hole carriers at the surface.

Nano Letters ASAP November 1, 2010

Ice Lithography for Nanodevices

Anpan Han†, Dimitar Vlassarev†, Jenny Wang†, Jene A. Golovchenko†‡, and Daniel Branton

We report the successful application of a new approach, ice lithography (IL), to fabricate nanoscale devices. The entire IL process takes place inside a modified scanning electron microscope (SEM), where a vapor-deposited film of water ice serves as a resist for e-beam lithography, greatly simplifying and streamlining device fabrication. We show that labile nanostructures such as carbon nanotubes can be safely imaged in an SEM when coated in ice. The ice film is patterned at high e-beam intensity and serves as a mask for lift-off without the device degradation and contamination associated with e-beam imaging and polymer resist residues. We demonstrate the IL preparation of carbon nanotube field effect transistors with high-quality trans-conductance properties.

Nano Letter ASAP November 1, 2010

Nanoelectronics: Nanoribbons on the edge

John A. Rogers

Arrays of graphene nanoribbons are fabricated on structured silicon carbide substrates using self-organized growth, without lithography and with well-controlled widths.

Graphene is attractive for electronics because of its exceptional properties and the relative ease with which it can be integrated into transistors and circuits. In the form of large-area planar sheets, it can be processed using straightforward adaptations of methods that are already in widespread use by the semiconductor industry.

Nature Nanotechnology News and Views 6 October 2010

Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets

M. Mannini, F. Pineider, C. Danieli, F. Totti, L. Sorace, Ph. Sainctavit, M.-A. Arrio, E. Otero, L. Joly, J. C. Cezar, A. Cornia

A fundamental step towards atomic- or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface1, or of a small paramagnetic molecule embedded in a nanojunction2, can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets3, 4, 5, 6 (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface7 was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices8, 9. Here we show that a preferential orientation of Fe4 complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs—their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization5, 6—can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.

Nature online 27 October 2010

Quantum physics: Tripping the light fantastic

Geoff Pryde about the book "Dance of Photon: From Einstein to Quantum Teleportation" by Anton Zeilinger

The only way to understand the quantum world is to measure it. This empirical view is dear to the heart of Anton Zeilinger, now at the University of Vienna, a leading figure in quantum physics through his work on correlated photons. In Dance of the Photons, he explores the phenomenon of quantum entanglement, the quantum correlations in the properties of particles.

...

Dance of the Photons is an enjoyable introduction to the strange world of quantum phenomena and the technologies they empower. It gives a foundation from which to ponder the nature of randomness and reality — and whether, in Vienna, the photon dance is performed to a Strauss waltz. Maybe Rupert can tell us over a lager, if he's ever allowed out of the tunnels.

Nature online 27 October 2010

Okt. 14. - okt. 20. (2010)

Válogatta: Bordács Sándor

Raman Scattering at Pure Graphene Zigzag Edges

Benjamin Krauss, Péter Nemes-Incze, Viera Skakalova, László P. Biro, Klaus von Klitzing, and Jurgen H. Smet

Theory has predicted rich and very distinct physics for graphene devices with boundaries that follow either the armchair or the zigzag crystallographic directions. A prerequisite to disclose this physics in experiment is to be able to produce devices with boundaries of pure chirality. Exfoliated flakes frequently exhibit corners with an odd multiple of 30°, which raised expectations that their boundaries follow pure zigzag and armchair directions. The predicted Raman behavior at such crystallographic edges however failed to confirm pure edge chirality. Here, we perform confocal Raman spectroscopy on hexagonal holes obtained after the anisotropic etching of prepatterned pits using carbothermal decomposition of SiO2. The boundaries of the hexagonal holes are aligned along the zigzag crystallographic direction and leave hardly any signature in the Raman map indicating unprecedented purity of the edge chirality. This work offers the first opportunity to experimentally confirm the validity of the Raman theory for graphene edges.

Nano Letters ASAP (2010)

Selective darkening of degenerate transitions demonstrated with two superconducting quantum bits

P. C. de Groot, J. Lisenfeld, R. N. Schouten, S. Ashhab, A. Lupaşcu, C. J. P. M. Harmans & J. E. Mooij

A new technique for controlling the quantum state of a superconducting qubit is now presented. Microwave pulses are applied in such a way that they excite only one of a pair of degenerate states. The concept enables construction of a controlled-NOT gate, a device important for quantum logic.

Nature Physics 6, 763 (2010)

Real-space mapping of magnetically quantized graphene states

David L. Miller, Kevin D. Kubista, Gregory M. Rutter, Ming Ruan, Walt A. de Heer, Markus Kindermann, Phillip N. First & Joseph A. Stroscio

Real-space mapping of the quantum Hall state at the Dirac point in epitaxial graphene reveals unexpected localized lifting of the degeneracy of this level. This could be the result of moiré interference caused by the twisting of the top layer with respect to underlying layers, suggesting possible new ways to understand and control the unusual properties of graphene.

Nature Physics 6, 811 (2010)

Andreev reflection between a normal metal and the FFLO superconductor II: a self-consistent approach

J. Kaczmarczyk, M. Sadzikowski, J. Spałek

We consider Andreev reflection in a two dimensional junction between a normal metal and a heavy fermion superconductor in the Fulde-Ferrell (FF) type of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We assume s-wave symmetry of the superconducting gap. The parameters of the superconductor: the gap magnitude, the chemical potential, and the Cooper pair center-of-mass momentum Q, are all determined self-consistently within a mean-field (BCS) scheme. The Cooper pair momentum Q is chosen as perpendicular to the junction interface. We calculate the junction conductance for a series of barrier strengths. In the case of incoming electron with spin \sigma = 1 only for magnetic fields close to the upper critical field H_{c2}, we obtain the so-called Andreev window i.e. the energy interval in which the reflection probability is maximal, which in turn is indicated by a peak in the conductance. The last result differs with other non-self-consistent calculations existing in the literature.

arXiv:1010.4136

Low-field magnetoelectric effect at room temperature

Yutaro Kitagawa, Yuji Hiraoka, Takashi Honda, Taishi Ishikura, Hiroyuki Nakamura & Tsuyoshi Kimura

Only few magnetoelectric materials, where magnetism and ferroelectricity are coupled, are known to exist at room temperature, and in most cases the magnetoelectric coupling is weak. The discovery of strong room-temperature magnetoelectric coupling in Sr3Co2Fe24O41 at low magnetic fields is therefore a significant advance towards the practical application of multiferroics.

Nature Materials 9, 797 (2010)

Two-dimensional superconductivity at a Mott insulator/band insulator interface LaTiO3/SrTiO3

J. Biscaras , N. Bergeal , A. Kushwaha , T. Wolf , A. Rastogi , R.C. Budhani & J. Lesueur

Transition metal oxides show a great variety of quantum electronic behaviours where correlations often have an important role. The achievement of high-quality epitaxial interfaces involving such materials gives a unique opportunity to engineer artificial structures where new electronic orders take place. One of the most striking result in this area is the recent observation of a two-dimensional electron gas at the interface between a strongly correlated Mott insulator LaTiO3 and a band insulator SrTiO3. The mechanism responsible for such a behaviour is still under debate. In particular, the influence of the nature of the insulator has to be clarified. In this article, we show that despite the expected electronic correlations, LaTiO3/SrTiO3 heterostructures undergo a superconducting transition at a critical temperature Tconset~300 mK. We have found that the superconducting electron gas is confined over a typical thickness of 12 nm and is located mostly on the SrTiO3 substrate.

Nature Communications 1, (2010)

Chiral Charge-Density Waves

J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda

We discovered the chirality of charge-density waves (CDW) in 1T-TiSe2 by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes Ia1∶Ia2∶Ia3=1∶0.7±0.1∶0.5±0.1, where Iai (i=1,2,3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality HCDW≡q1•(q2×q3), where qi are the CDW q vectors, which is independent of the symmetry of components. The nonzero HCDW—the triple-q vectors do not exist in an identical plane in the reciprocal space—should induce a real-space chirality in CDW system.

Physical Review Letters 105, 176401 (2010)

Theory of electric polarization in multi-orbital Mott insulators

Maxim Mostovoy, Kentaro Numura, Naoto Nagaosa

The interaction between the electric field, E, and spins in multi-orbital Mott insulators is studied theoretically. We find a generic dynamical coupling mechanism, which works for all crystal lattices and which does not involve relativistic effects. The general form of the coupling is -T_ab E_a e_b, where e is the `internal' electric field originating from the dynamical Berry phase of electrons and T_ab is a tensor determined by lattice symmetry. We discuss several effects of this interaction: (i) an unusual electron spin resonance induced by an oscillating electric field, (ii) the displacement of spin textures in an applied electric field, and (iii) the resonant absorption of circularly polarized light by Skyrmions, magnetic bubbles, and magnetic vortices.

arXiv:1010.3687v1

Okt. 7. - okt. 13. (2010)

Válogatta: Makk Péter

An All-Electric Single-Molecule Motor

Johannes S. Seldenthuis, Ferry Prins, Joseph M. Thijssen, and Herre S. J. van der Zant

Many types of molecular motors have been proposed and synthesized in recent years, displaying different kinds of motion, and fueled by different driving forces such as light, heat, or chemical reactions. We propose a new type of molecular motor based on electric field actuation and electric current detection of the rotational motion of a molecular dipole embedded in a three-terminal single-molecule device. The key aspect of this all-electronic design is the conjugated backbone of the molecule, which simultaneously provides the potential landscape of the rotor orientation and a real-time measure of that orientation through the modulation of the conductivity. Using quantum chemistry calculations, we show that this approach provides full control over the speed and continuity of motion, thereby combining electrical and mechanical control at the molecular level over a wide range of temperatures. Moreover, chemistry can be used to change all key parameters of the device, enabling a variety of new experiments on molecular motors.

ACS Nano ASAP(2010)

Time-resolved detection of spin-transfer-driven ferromagnetic resonance and spin torque measurement in magnetic tunnel junctions

Chen Wang, Yong-Tao Cui, Jordan A. Katine, Robert A. Buhrman, Daniel C. Ralph

Several experimental techniques have been introduced in recent years in attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs). The dependence of spin torque on bias is important for understanding fundamental spin physics in magnetic devices and for applications. However, previous techniques have provided only indirect measures of the torque and their results to date for the bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can be measured directly by using time-domain techniques to detect resonant magnetic precession in response to an oscillating spin torque. The technique is accurate in the high-bias regime relevant for applications, and because it detects directly small-angle linear-response magnetic dynamics caused by spin torque it is relatively immune to artifacts affecting competing techniques. At high bias we find that the spin torque vector differs markedly from the simple lowest-order Taylor series approximations commonly assumed.

arXiv:1010.1777v1

Gate-Defined Graphene Quantum Point Contact in the Quantum Hall Regime

S. Nakaharai, J. R. Williams, C. M. Marcus

We investigate transport in a gate-defined graphene quantum point contact in the quantum Hall regime. Edge states confined to the interface of p and n regions in the graphene sheet are controllably brought together from opposite sides of the sample and allowed to mix in this split-gate geometry. Among the expected quantum Hall features, an unexpected additional plateau at 0.5 h/e^2 is observed. We propose that chaotic mixing of edge channels gives rise to the extra plateau.

arXiv:1010.1919v1

Spin-polarization of platinum (111) induced by the proximity to cobalt nanostripes

Focko Meier, Samir Lounis, Jens Wiebe, Lihui Zhou, Swantje Heers, Phivos Mavropoulos, Peter H. Dederichs, Stefan Blügel, Roland Wiesendanger

We measured a spin polarization above a Pt (111) surface in the vicinity of a Co nanostripe by spin-polarized scanning tunneling spectroscopy. The spin polarization is exponentially decaying away from the Pt/Co interface and is detectable at distances larger than 1 nm. By performing self-consistent ab-initio calculations of the electronic-structure for a related model system we reveal the interplay between the induced magnetic moments within the Pt surface and the spin-resolved electronic density of states above the surface.

arXiv:1010.2109v1

Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene

R. Thomas Weitz, Monica T. Allen, Benjamin E. Feldman, Jens Martin, Amir Yacoby

The non-interacting energy spectrum of graphene and its bilayer counterpart consists of multiple degeneracies owing to the inherent spin, valley and layer symmetries. Interactions among charge carriers are expected to spontaneously break these symmetries, leading to gapped ordered states. In the quantum Hall regime these states are predicted to be ferromagnetic in nature whereby the system becomes spin polarized, layer polarized or both. In bilayer graphene, due to its parabolic dispersion, interaction-induced symmetry breaking is already expected at zero magnetic field. In this work, the underlying order of the various broken-symmetry states is investigated in bilayer graphene that is suspended between top and bottom gate electrodes. By controllably breaking the spin and sublattice symmetries we are able to deduce the order parameter of the various quantum Hall ferromagnetic states. At small carrier densities, we identify for the first time three distinct broken symmetry states, one of which is consistent with either spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry.

arXiv:1010.0989v1

Measurement of the \nu= 1/3 fractional quantum Hall energy gap in suspended graphene

Fereshte Ghahari, Yue Zhao, Paul Cadden-Zimansky, Kirill Bolotin, Philip Kim

We report on magnetotransport measurements of multi-terminal suspended graphene devices. Fully developed integer quantum Hall states appear in magnetic fields as low as 2 T. At higher fields the formation of longitudinal resistance minima and transverse resistance plateaus are seen corresponding to fractional quantum Hall states, most strongly for \nu= 1/3. By measuring the temperature dependence of these resistance minima, the energy gap for the 1/3 fractional state in graphene is determined to be at ~20 K at 14 T. This gap is at least 3 times larger than the observed gaps for the corresponding state in the best quality semiconductor heterostructures.

arXiv:1010.1187v1

Multicomponent fractional quantum Hall effect in graphene

C.R. Dean, A.F. Young, P. Cadden-Zimansky, L. Wang, H. Ren, K. Watanabe, T. Taniguchi, P. Kim, J. Hone, K.L. Shepard

We report observation of the fractional quantum Hall effect (FQHE) in high mobility multi-terminal graphene devices, fabricated on a single crystal boron nitride substrate. We observe an unexpected hierarchy in the emergent FQHE states that may be explained by strongly interacting composite Fermions with full SU(4) symmetric underlying degrees of freedom. The FQHE gaps are measured from temperature dependent transport to be up 10 times larger than in any other semiconductor system. The remarkable strength and unusual hierarcy of the FQHE described here provides a unique opportunity to probe correlated behavior in the presence of expanded quantum degrees of freedom.

arXiv:1010.1179v1

Mean-field quantum phase transition in graphene and in general gapless systems

Ádám Bácsi, Attila Virosztek, László Borda, and Balázs Dóra

We study the quantum-critical properties of antiferromagnetism in graphene at T=0 within mean-field (MF) theory. The resulting exponents differ from the conventional MF exponents, describing finite-temperature transitions. Motivated by this, we have developed the MF theory of general gapless phases with density of states ρ(ε)∼|ε|r, r>−1, with the interaction as control parameter. For r>2, the conventional MF exponents à la Landau are recovered, while for −1<r<2, the exponents vary significantly with r. The critical interaction is finite for r>0, therefore no weak-coupling solution exists in this range. This generalizes the results on quantum criticality of the gapless Kondo systems to bulk correlated phases.

Phys. Rev. B 82, 153406 (2010)

Josephson current in ballistic superconductor-graphene systems

Imre Hagymási, Andor Kormányos, and József Cserti

We calculate the phase, the temperature and the junction length dependence of the supercurrent for ballistic graphene Josephson junctions. For low temperatures we find nonsinusoidal dependence of the supercurrent on the superconductor phase difference for both short and long junctions. The skewness, which characterizes the deviaton of the current-phase relation from a simple sinusoidal one, shows a linear dependence on the critical current for small currents. We discuss the similarities and differences with respect to the classical theory of Josephson junctions, where the weak link is formed by a diffusive or ballistic metal. The relation to other recent theoretical results on graphene Josephson junctions is pointed out and the possible experimental relevance of our work is considered as well.

Phys. Rev. B 82, 134516 (2010)

Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition

Seunghyun Lee, Kyunghoon Lee, and Zhaohui Zhong

The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm2 size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices.

Nano Lett.,, Article ASAP, DOI: 10.1021/nl1029978

The Relation between Structure and Quantum Interference in Single Molecule Junctions

Troels Markussen, Robert Stadler, and Kristian S. Thygesen

Quantum interference (QI) of electron pathways has recently attracted increased interest as an enabling tool for single-molecule electronic devices. Although various molecular systems have been shown to exhibit QI effects and a number of methods have been proposed for its analysis, simple guidelines linking the molecular structure to QI effects in the phase-coherent transport regime have until now been lacking. In the present work we demonstrate that QI in aromatic molecules is intimately related to the topology of the molecule’s π system and establish a simple graphical scheme to predict the existence of QI-induced transmission antiresonances. The generality of the scheme, which is exact for a certain class of tight-binding models, is proved by a comparison to first-principles transport calculations for 10 different configurations of anthraquinone as well as a set of cross-conjugated molecular wires.

Nano Lett., Article ASAP, DOI: 10.1021/nl101688a

Hybrid superconductor–quantum dot devices

Silvano De Franceschi, Leo Kouwenhoven, Christian Schönenberger & Wolfgang Wernsdorfer

Advances in nanofabrication techniques have made it possible to make devices in which superconducting electrodes are connected to non-superconducting nanostructures such as quantum dots. The properties of these hybrid devices result from a combination of a macroscopic quantum phenomenon involving large numbers of electrons (superconductivity) and the ability to control single electrons, offered by quantum dots. Here we review research into electron transport and other fundamental processes that have been studied in these devices. We also describe potential applications, such as a transistor in which the direction of a supercurrent can be reversed by adding just one electron to a quantum dot.

Nature Nanotechnology 5, 703 - 711 (2010)

Boron nitride substrates for high-quality graphene electronics

C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard & J. Hone

Graphene devices on standard SiO2 substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO2. These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.

Nature Nanotechnology 5,722–726 (2010)

Direct observation of single-charge-detection capability of nanowire field-effect transistors

J. Salfi I. G. Savelyev, M. Blumin, S. V. Nair & H. E. Ruda

A single localized charge can quench the luminescence of a semiconductor nanowire, but relatively little is known about the effect of single charges on the conductance of the nanowire. In one-dimensional nanostructures embedded in a material with a low dielectric permittivity, the Coulomb interaction and excitonic binding energy are much larger than the corresponding values when embedded in a material with the same dielectric permittivity. The stronger Coulomb interaction is also predicted to limit the carrier mobility in nanowires. Here, we experimentally isolate and study the effect of individual localized electrons on carrier transport in InAs nanowire field-effect transistors, and extract the equivalent charge sensitivity. In the low carrier density regime, the electrostatic potential produced by one electron can create an insulating weak link in an otherwise conducting nanowire field-effect transistor, modulating its conductance by as much as 4,200% at 31 K. The equivalent charge sensitivity, 4 × 10−5 e Hz−1/2 at 25 K and 6 × 10−5 e Hz−1/2 at 198 K, is orders of magnitude better than conventional field-effect transistors5 and nanoelectromechanical systems, and is just a factor of 20–30 away from the record sensitivity for state-of-the-art single-electron transistors operating below 4 K (ref. 8). This work demonstrates the feasibility of nanowire-based single-electron memories and illustrates a physical process of potential relevance for high performance chemical sensors. The charge-state-detection capability we demonstrate also makes the nanowire field-effect transistor a promising host system for impurities (which may be introduced intentionally or unintentionally) with potentially long spin lifetimes, because such transistors offer more sensitive spin-to-charge conversion readout than schemes based on conventional field-effect transistors.

Nature Nanotechnology 5, 737 - 741 (2010)

Hybrid InAs nanowire-vanadium proximity SQUID

Panayotis Spathis, Subhajit Biswas, Stefano Roddaro, L.Sorba, F.Giazotto, Fabio Beltram

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) based on InAs nanowires and vanadium superconducting electrodes. These mesoscopic devices are found to be extremely robust against thermal cycling and to operate up to temperatures of $\sim2.5$~K with reduced power dissipation. We show that our geometry allows to obtain nearly-symmetric devices with very large magnetic-field modulation of the critical current. All these properties make these devices attractive for on-chip quantum-circuit implementation.

arXiv:1010.2545v1

Chemical Functionalization of Graphene Enabled by Phage Displayed Peptides

Yue Cui†, Sang N. Kim, Sharon E. Jones, L.L. Wissler, Rajesh R. Naik, and Michael C. McAlpine

The development of a general approach for the nondestructive chemical and biological functionalization of graphene could expand opportunities for graphene in both fundamental studies and a variety of device platforms. Graphene is a delicate single-layer, two-dimensional network of carbon atoms whose properties can be affected by covalent modification. One method for functionalizing materials without fundamentally changing their inherent structure is using biorecognition moieties. In particular, oligopeptides are molecules containing a broad chemical diversity that can be achieved within a relatively compact size. Phage display is a dominant method for identifying peptides that possess enhanced selectivity toward a particular target. Here, we demonstrate a powerful yet benign approach for chemical functionalization of graphene via comprehensively screened phage displayed peptides. Our results show that graphene can be selectively recognized even in nanometer-defined strips. Further, modification of graphene with bifunctional peptides reveals both the ability to impart selective recognition of gold nanoparticles and the development of an ultrasensitive graphene-based TNT sensor. We anticipate that these results could open exciting opportunities in the use of graphene in fundamental biochemical recognition studies, as well as applications ranging from sensors to energy storage devices.

Nano Lett., Article ASAP, DOI: 10.1021/nl102564d

Szept. 30. - okt. 6. (2010)

Válogatta: Geresdi Attila

Spin Seebeck insulator

K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E.W. Bauer, S. Maekawa and E. Saitoh

Thermoelectric generation is an essential function in future energy-saving technologies. However, it has so far been an exclusive feature of electric conductors, a situation which limits its application; conduction electrons are often problematic in the thermal design of devices. Here we report electric voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, the magnetic insulator LaY2Fe5O12 can convert a heat flow into a spin voltage. Attached Pt films can then transform this spin voltage into an electric voltage as a result of the inverse spin Hall effect. The experimental results require us to introduce a thermally activated interface spin exchange between LaY2Fe5O12 and Pt. Our findings extend the range of potential materials for thermoelectric applications and provide a crucial piece of information for understanding the physics of the spin Seebeck effect.

Nature Materials, DOI:10.1038/NMAT2856

Nobel Prize 2010: Geim and Novoselov

Ed Gerstner

The 2010 Nobel Prize in Physics has been awarded to Andre Geim and Konstantin Novoselov, “for groundbreaking experiments regarding the two-dimensional material graphene”.

Nature Physics, DOI:10.1038/nphys1836

Electrical Measurement of the Direct Spin Hall Effect in Fe/InxGa1-xAs Heterostructures

E. S. Garlid, Q. O. Hu, M. K. Chan, C. J. Palmstrøm, and P. A. Crowell

We report on an all-electrical measurement of the spin Hall effect in epitaxial Fe/InxGa1-xAs heterostructures with n-type (Si) channel doping and highly doped Schottky tunnel barriers. A transverse spin current generated by an ordinary charge current flowing in the InxGa1_xAs is detected by measuring the spin accumulation at the edges of the channel. The spin accumulation is identified through the observation of a Hanle effect in the voltage measured by pairs of ferromagnetic Hall contacts. We investigate the bias and temperature dependence of the resulting Hanle signal and determine the skew and side-jump contributions to the total spin Hall conductivity.

Phys. Rev. Letters, DOI:10.1103/PhysRevLett.105.156602

Fast domain wall motion in magnetic comb structures

E. R. Lewis, D. Petit, L. O’Brien, A. Fernandez-Pacheco, J. Sampaio, A-V. Jausovec, H. T. Zeng, D. E. Read and R. P. Cowburn

Modern fabrication technology has enabled the study of submicron ferromagnetic strips with a particularly simple domain structure, allowing single, well-defined domain walls to be isolated and characterized. However, these domain walls have complex field-driven dynamics. The wall velocity initially increases with field, but above a certain threshold the domain wall abruptly slows down, accompanied by periodic transformations of the domain wall structure. This behaviour is potentially detrimental to the speed and proper functioning of proposed domain-wall-based devices, and although methods for suppression of the breakdown have been demonstrated in simulations, a convincing experimental demonstration is lacking. Here, we show experimentally that a series of crossshaped traps acts to prevent transformations of the domain wall structure and increase the domain wall velocity by a factor of four compared to the maximum velocity on a plain strip. Our results suggest a route to faster and more reliable domain wall devices for memory, logic and sensing.

Nature Materials, DOI:10.1038/NMAT2857

Nanomaterials: Graphene rests easy

R. Thomas Weitz & Amir Yacoby

Samples of graphene supported on boron nitride demonstrate superior electrical properties, achieving levels of performance that are comparable to those observed with suspended samples.

Nature Nano, DOI:10.1038/nnano.2010.201

Nanoelectronics: Nanoribbons on the edge

John A. Rogers

Arrays of graphene nanoribbons are fabricated on structured silicon carbide substrates using self-organized growth, without lithography and with well-controlled widths.

Nature Nano, DOI:10.1038/nnano.2010.200

Nanopores: Graphene opens up to DNA

Zuzanna S. Siwy & Matthew Davenport

It might be possible to sequence DNA by passing the molecule through a small hole in a sheet of graphene. The nanopores that are found in the membranes of cells have an important role in biology and, along with artificial nanopores fabricated in materials such as silicon nitride, they have also been used to detect single molecules and, in some cases, extract more detailed information such as the size and shape of the molecules1. The capabilities of these systems are influenced by the diameter of the nanopore and also by the thickness of the membrane that contains it.

Nature Nano, DOI:10.1038/nnano.2010.198

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Recent interesting articles

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