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Cond-mat Mesoscale and Nanoscale Physics - recent papers

Physical Review Letters

Physical Review B

Ápr. 14. - Ápr. 21. (2011)

Válogatta: Oroszlány László

Magnetism-Dependent Transport Phenomena in Hydrogenated Graphene: From Spin-Splitting to Localization Effects

Nicolas Leconte, David Soriano, Stephan Roche, Pablo Ordejon, Jean-Christophe Charlier and J. J. Palacios

Spin-dependent transport in hydrogenated two-dimensional graphene is explored theoretically. Adsorbed atomic hydrogen impurities can either induce a localantiferromagnetic, ferromagnetic, or nonmagnetic state depending on their density and relative distribution. To describe the various magnetic possibilities of hydrogenated graphene, a self-consistent Hubbard Hamiltonian, optimized by ab initio calculations, is first solved in the mean field approximation for small graphene cells. Then, an efficient order N Kubo transport methodology is implemented, enabling large scale simulations of functionalized graphene. Depending on the underlying intrinsic magnetic ordering of hydrogen-induced spins, remarkably different transport features are predicted for the same impurity concentration. Indeed, while the disordered nonmagnetic graphene system exhibits a transition from diffusive to localization regimes, the intrinsic ferromagnetic state exhibits unprecedented robustness toward quantum interference, maintaining, for certain resonant energies, a quasiballistic regime up to the micrometer scale. Consequently, low temperature transport measurements could unveil the presence of a magnetic state in weakly hydrogenated graphene.

http://pubs.acs.org/doi/abs/10.1021/nn200558d

Raman Spectroscopy and in Situ Raman Spectroelectrochemistry of Bilayer 12C/13C Graphene

Martin Kalbac, Hootan Farhat, Jing Kong, Pavel Janda, Ladislav Kavan and Mildred S. Dresselhaus

Bilayer graphene was prepared by the subsequent deposition of a 13C single-layer graphene and a 12C single-layer graphene on top of a SiO2/Si substrate. The bilayer graphene thus prepared was studied using Raman spectroscopy and in situ Raman spectroelectrochemistry. The Raman frequencies of the 13C graphene bands are significantly shifted with respect to those of 12C graphene, which allows us to investigate the single layer components of bilayer graphene individually. It is shown that the bottom layer of the bilayer graphene is significantly doped from the substrate, while the top layer does not exhibit a signature of the doping from the environment. The electrochemical doping has the same effect on the charge carrier concentration at the top and the bottom layer despite the top layer being the only layer in contact with the electrolyte. This is here demonstrated by essentially the same frequency shifts of the G and G′ bands as a function of the electrode potential for both the top and bottom layers. Nevertheless, analysis of the intensity of the Raman modes showed an anomalous bleaching of the Raman intensity of the G mode with increasing electrode potential, which was not observed previously in one-layer graphene.

http://pubs.acs.org/doi/abs/10.1021%2Fnl2001956

New Synthesis Method for the Growth of Epitaxial Graphene

Xiaozhu Yu, Choongyu Hwang, Chris M. Jozwiak, Annemarie Kohl, Andreas K. Schmid, Alessandra Lanzara

As a viable candidate for an all-carbon post-CMOS electronics revolution, epitaxial graphene has attracted significant attention. To realize its application potential, reliable methods for fabricating large-area single-crystalline graphene domains are required. A new way to synthesize high quality epitaxial graphene, namely "face-to-face" method, has been reported in this paper. The structure and morphologies of the samples are characterized by low-energy electron diffraction, atomic force microscopy, angle-resolved photoemission spectroscopy and Raman spectroscopy. The grown samples show better quality and larger length scales than samples grown through conventional thermal desorption. Moreover the graphene thickness can be easily controlled by changing annealing temperature.

http://arxiv.org/abs/1104.3907

Control of Thermal and Electronic Transport in Defect-Engineered Graphene Nanoribbons

Justin Haskins, Alper Kınacı, Cem Sevik, Hâldun Sevinçli, Gianaurelio Cuniberti and Tahir Çağın

The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and non-equilibrium Green’s function methods. A variety of randomly oriented and distributed defects, single and double vacancies, Stone−Wales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (>100 nm long and >15 nm wide). We observe substantial reduction in thermal conductivity due to all forms of defects, whereas electrical conductance reveals a peculiar defect-type-dependent response. We find that a 0.1% single vacancy concentration and a 0.23% double vacancy or Stone−Wales concentration lead to a drastic reduction in thermal conductivity of GNRs, namely, an 80% reduction from the pristine one of the same width. Edge roughness with an rms value of 7.28 Å leads to a similar reduction in thermal conductivity. Randomly distributed bulk vacancies are also found to strongly suppress the ballistic nature of electrons and reduce the conductance by 2 orders of magnitude. However, we have identified that defects close to the edges and relatively small values of edge roughness preserve the quasi-ballistic nature of electronic transport. This presents a route of independently controlling electrical and thermal transport by judicious engineering of the defect distribution; we discuss the implications of this for thermoelectric performance.

http://pubs.acs.org/doi/abs/10.1021/nn200114p

Imaging Universal Conductance Fluctuations in Graphene

Mario F. Borunda, Jesse Berezovsky, Robert M. Westervelt, and Eric J. Heller

We study conductance fluctuations (CF) and the sensitivity of the conductance to the motion of a single scatterer in two-dimensional massless Dirac systems. Our extensive numerical study finds limits to the predicted universal value of CF. We find that CF are suppressed for ballistic systems near the Dirac point and approach the universal value at sufficiently strong disorder. The conductance of massless Dirac fermions is sensitive to the motion of a single scatterer. CF of order e2/h result from the motion of a single impurity by a distance comparable to the Fermi wavelength. This result applies to graphene systems with a broad range of impurity strength and concentration while the dependence on the Fermi wavelength can be explored via gate voltages. Our prediction can be tested by comparing graphene samples with varying amounts of disorder and can be used to understand interference effects in mesoscopic graphene devices.

http://pubs.acs.org/doi/abs/10.1021/nn103450d

Giant Nonlocality Near the Dirac Point in Graphene

D. A. Abanin, S. V. Morozov, L. A. Ponomarenko, R. V. Gorbachev, A. S. Mayorov, M. I. Katsnelson, K. Watanabe, T. Taniguchi, K. S. Novoselov, L. S. Levitov, A. K. Geim

Transport measurements have been a powerful tool for discovering electronic phenomena in graphene. We report nonlocal measurements performed in the Hall bar geometry with voltage probes far away from the classical path of charge flow. We observed a large nonlocal response near the Dirac point in fields as low as 0.1 tesla, which persisted up to room temperature. The nonlocality is consistent with the long-range flavor currents induced by the lifting of spin/valley degeneracy. The effect is expected to contribute strongly to all magnetotransport phenomena near the neutrality point.

http://www.sciencemag.org/content/332/6027/328.abstract

Bottom-gated epitaxial graphene

Daniel Waldmann, Johannes Jobst, Florian Speck, Thomas Seyller, Michael Krieger, Heiko B. Weber

High-quality epitaxial graphene on silicon carbide (SiC) is today available in wafer size1, 2. Similar to exfoliated graphene3, 4, its charge carriers are governed by the Dirac–Weyl Hamiltonian5, 6, 7 and it shows excellent mobilities7, 8. For many experiments with graphene, in particular for surface science, a bottom gate is desirable9, 10, 11, 12, 13. Commonly, exfoliated graphene flakes are placed on an oxidized silicon wafer that readily provides a bottom gate3, 4, 11. However, this cannot be applied to epitaxial graphene as the SiC provides the source material out of which graphene grows. Here, we present a reliable scheme for the fabrication of bottom-gated epitaxial graphene devices, which is based on nitrogen (N) implantation into a SiC wafer and subsequent graphene growth. We demonstrate working devices in a broad temperature range from 6 to 300 K. Two gating regimes can be addressed, which opens a wide engineering space for tailored devices by controlling the doping of the gate structure.

http://www.nature.com/nmat/journal/v10/n5/full/nmat2988.html

Drude conductivity of Dirac fermions in graphene

Jason Horng, Chi-Fan Chen, Baisong Geng, Caglar Girit, Yuanbo Zhang, Zhao Hao, Hans A. Bechtel, Michael Martin, Alex Zettl, Michael F. Crommie, Y. Ron Shen, and Feng Wang

Electrons moving in graphene behave as massless Dirac fermions, and they exhibit fascinating low-frequency electrical transport phenomena. Their dynamic response, however, is little known at frequencies above one terahertz (THz). Such knowledge is important not only for a deeper understanding of the Dirac electron quantum transport, but also for graphene applications in ultrahigh-speed THz electronics and infrared (IR) optoelectronics. In this paper, we report measurements of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations. The conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies, but its absolute strength is lower than theoretical predictions. This anomalous reduction of free-electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum rule.

http://prb.aps.org/abstract/PRB/v83/i16/e165113

Acoustic phonons in graphene nanoribbons

Matthias Droth, Guido Burkard

Phonons are responsible for limiting both the carrier mobility and the spin relaxation time. In view of a possible transistor function as well as spintronics applications in graphene nanoribbons, we present a theoretical study of acoustic phonons in these nanostructures. Using a two-dimensional continuum model which takes into accout the monatomic thickness of graphene, we derive hermitian wave equations as well as phonon creation and annihilation operators, thus allowing for a quantum mechanical treatment of the system. Two types of boundary configurations, which we believe can be realized in experiment, are discussed: (i) fixed and (ii) free boundaries. The former leads to a gapped phonon dispersion relation while the latter exhibits an ungapped dispersion relation and a finite sound velocity of out-of-plane modes at the center of the Brillouin zone. In the limit of negligible boundary effects, bulk-like behaviour is restored.

http://arxiv.org/abs/1104.3520

Localized Andreev edge states in HgTe quantum wells.

I.M. Khaymovich, N.M. Chtchelkatchev, V.M. Vinokur

We investigate the interplay of superconductivity and the topological order in the topological insulator (TI) formed in HgTe-CdTe quantum wells coupled to the s-wave isotropic superconductor (SC) placed on top of CdTe layer. Proximity effect induces superconducting correlations in TI effecting classification of the TI electronic states. Depending on the strength of the coupling and the initial chemical potentials in SC and TI, the edge states either (i) become gapped, or (ii) hybridize into localized Andreev states, or else (iii) merge with the bulk states turning the TI into an anisotropic narrow-gap semiconductor.

arXiv:1104.3499v1

Quantum Hall Effect in Thin Films of Three-Dimensional Topological Insulators.

Huichao Li, L. Sheng, D. Y. Xing

We show that a thin film of a three-dimensional topological insulator (3DTI) with an exchange field is a realization of the famous Haldane model for quantum Hall effect (QHE) without Landau levels. The exchange field plays the role of staggered fluxes on the honeycomb lattice, and the hybridization gap of the surface states is equivalent to alternating on-site energies on the AB sublattices. A peculiar phase diagram for the QHE is predicted in 3DTI thin films under an applied magnetic field, which is quite different from that either in traditional QHE systems or in graphene.

arXiv:1104.3407v1

Helical States of Topological Insulator Bi2Se3

Yonghong Zhao, Yibin Hu, Lei Liu, Yu Zhu, and Hong Guo

We report density functional theory analysis of the electronic and quantum transport properties of Bi2Se3 topological insulator, focusing on the helical surface states at the Fermi level EF. The calculated Dirac point and the tilt angle of the electron spin in the helical states are compared quantitatively with the experimental data. The calculated conductance near EF shows a V-shaped spectrum, consistent with STM measurements. The spins in the helical states at EF not only tilts out of the two-dimensional plane, they also oscillate with a 3-fold symmetry going around the two-dimensional Brillouin zone. The helical states penetrate into the material bulk, where the first quintuple layer contributes 70% of the helical wave functions.

http://pubs.acs.org/doi/abs/10.1021/nl200584f

Observation of topological order in a Superconducting doped topological insulator (based on the Bi2Se3 class).

L. Andrew Wray, Suyang Xu, Yuqi Xia, Dong Qian, Alexei V. Fedorov, Hsin Lin, Arun Bansil, Yew San Hor, Robert J. Cava, M. Zahid Hasan

Topological insulators embody a new state of matter characterized entirely by the topological invariants of the bulk electronic structure rather than any form of spontaneously broken symmetry. Unlike the 2D quantum Hall or quantum spin-Hall-like systems, the three dimensional (3D) topological insulators can host magnetism and superconductivity which has generated widespread research activity in condensed-matter and materials-physics communities. Thus there is an explosion of interest in understanding the rich interplay between topological and the broken-symmetry states (such as superconductivity), greatly spurred by proposals that superconductivity introduced into certain band structures will host exotic quasiparticles which are of interest in quantum information science. The observations of superconductivity in doped Bi_2Se_3 (Cu$_x$Bi$_2$Se$_3$) and doped Bi_2Te_3 (Pd$_x$-Bi$_2$Te$_3$ T$_c$ $\sim$ 5K) have raised many intriguing questions about the spin-orbit physics of these ternary complexes while any rigorous theory of superconductivity remains elusive. Here we present key measurements of electron dynamics in systematically tunable normal state of Cu$_x$Bi$_2$Se$_3$ (x=0 to 12%) gaining insights into its spin-orbit behavior and the topological nature of the surface where superconductivity takes place at low temperatures. Our data reveal that superconductivity occurs (in sample compositions) with electrons in a bulk relativistic kinematic regime and we identify that an unconventional doping mechanism causes the topological surface character of the undoped compound to be preserved at the Fermi level of the superconducting compound, where Cooper pairing occurs at low temperatures. These experimental observations provide important clues for developing a theory of topological-superconductivity in 3D topological insulators.

arXiv:1104.3881v1

Surface-Quantized Anomalous Hall Current and the Magnetoelectric Effect in Magnetically Disordered Topological Insulators

Kentaro Nomura and Naoto Nagaosa

We study theoretically the role of quenched magnetic disorder at the surface of topological insulators by numerical simulation and scaling analysis based on the massive Dirac fermion model. This addresses the problem of Anderson localization on chiral anomaly. It is found that all the surface states are localized, while the transverse conductivity is quantized to be ±e2/2h as long as the Fermi energy is within the bulk gap. This greatly facilitates the realization of the topological magnetoelectric effect proposed by Qi et al. [Phys. Rev. B 78, 195424 (2008)] with the surface magnetization direction being controlled by the simultaneous application of magnetic and electric fields.

http://prl.aps.org/abstract/PRL/v106/i16/e166802

Competing exotic topological insulator phases in transition-metal oxides on the pyrochlore lattice with distortion

Mehdi Kargarian, Jun Wen, and Gregory A. Fiete

In this work we investigate the phase diagram of heavy (4d and 5d) transition-metal oxides on the pyrochlore lattice, such as those of the form A2M2O7, where A is a rare-earth element and M is a transition-metal element. We focus on the competition between Coulomb interaction, spin-orbit coupling, and lattice distortion when these energy scales are comparable. Strong spin-orbit coupling entangles the spin and the t2g d orbitals giving rise to doublet j=1/2 and quadruplet j=3/2 states. In contrast to previous works which focused on the doublet manifold, we also discuss the quadruplet manifold which is relevant for several pyrochlore oxides. The Coulomb interaction is taken into account by use of the slave-rotor mean-field theory and different classes of lattice distortions which further split the levels of the quadruplet j=3/2 manifold are studied. Various topological phases are predicted, including exotic strong and weak topological Mott insulating phases. We discuss the general structure of the phase diagram for several values of d-shell filling and various symmetry classes of lattice distortions. Our results are relevant to the search for exotic topological insulators and quantum spin liquids in strongly correlated materials with strong spin-orbit coupling.

http://prb.aps.org/abstract/PRB/v83/i16/e165112

Quantum simulation of antiferromagnetic spin chains in an optical lattice

Jonathan Simon, Waseem S. Bakr, Ruichao Ma, M. Eric Tai, Philipp M. Preiss & Markus Greiner

Understanding exotic forms of magnetism in quantum mechanical systems is a central goal of modern condensed matter physics, with implications for systems ranging from high-temperature superconductors to spintronic devices. Simulating magnetic materials in the vicinity of a quantum phase transition is computationally intractable on classical computers, owing to the extreme complexity arising from quantum entanglement between the constituent magnetic spins. Here we use a degenerate Bose gas of rubidium atoms confined in an optical lattice to simulate a chain of interacting quantum Ising spins as they undergo a phase transition. Strong spin interactions are achieved through a site-occupation to pseudo-spin mapping. As we vary a magnetic field, quantum fluctuations drive a phase transition from a paramagnetic phase into an antiferromagnetic phase. In the paramagnetic phase, the interaction between the spins is overwhelmed by the applied field, which aligns the spins. In the antiferromagnetic phase, the interaction dominates and produces staggered magnetic ordering. Magnetic domain formation is observed through bothin situ site-resolved imaging and noise correlation measurements. By demonstrating a route to quantum magnetism in an optical lattice, this work should facilitate further investigations of magnetic models using ultracold atoms, thereby improving our understanding of real magnetic materials.

http://www.nature.com/nature/journal/v472/n7343/full/nature09994.html

Teleportation of Nonclassical Wave Packets of Light

Noriyuki Lee, Hugo Benichi, Yuishi Takeno, Shuntaro Takeda, James Webb, Elanor Huntington, Akira Furusawa

We report on the experimental quantum teleportation of strongly nonclassical wave packets of light. To perform this full quantum operation while preserving and retrieving the fragile nonclassicality of the input state, we have developed a broadband, zero-dispersion teleportation apparatus that works in conjunction with time-resolved state preparation equipment. Our approach brings within experimental reach a whole new set of hybrid protocols involving discrete- and continuous-variable techniques in quantum information processing for optical sciences.

http://www.sciencemag.org/content/332/6027/330.abstract

Infrared-spectroscopic nanoimaging with a thermal source

F. Huth, M. Schnell, J. Wittborn, N. Ocelic & R. HillenbrandFourier-transform infrared (FTIR) spectroscopy is a widely used analytical tool for chemical identification of inorganic, organic and biomedical materials, as well as for exploring conduction phenomena. Because of the diffraction limit, however, conventional FTIR cannot be applied for nanoscale imaging. Here we demonstrate a novel FTIR system that allows for infrared-spectroscopic nanoimaging of dielectric properties (nano-FTIR). Based on superfocusing of thermal radiation with an infrared antenna, detection of the scattered light, and strong signal enhancement employing an asymmetric FTIR spectrometer, we improve the spatial resolution of conventional infrared spectroscopy by more than two orders of magnitude. By mapping a semiconductor device, we demonstrate spectroscopic identification of silicon oxides and quantification of the free-carrier concentration in doped Si regions with a spatial resolution better than 100 nm. We envisage nano-FTIR becoming a powerful tool for chemical identification of nanomaterials, as well as for quantitative and contact-free measurement of the local free-carrier concentration and mobility in doped nanostructures.

http://www.nature.com/nmat/journal/v10/n5/abs/nmat3006.html

Environmental Control of Single-Molecule Junction Transport

V. Fatemi, M. Kamenetska, J. B. Neaton, and L. Venkataraman

The conductance of individual 1,4-benzenediamine (BDA)–Au molecular junctions is measured in different solvent environments using a scanning tunneling microscope based point-contact technique. Solvents are found to increase the conductance of these molecular junctions by as much as 50%. Using first principles calculations, we explain this increase by showing that a shift in the Au contact work function is induced by solvents binding to undercoordinated Au sites around the junction. Increasing the Au contact work function reduces the separation between the Au Fermi energy and the highest occupied molecular orbital of BDA in the junction, increasing the measured conductance. We demonstrate that the solvent-induced shift in conductance depends on the affinity of the solvent to Au binding sites and also on the induced dipole (relative to BDA) upon adsorption. Via this mechanism, molecular junction level alignment and transport properties can be statistically altered by solvent molecule binding to the contact surface.

http://pubs.acs.org/doi/abs/10.1021/nl200324e

Ápr. 1. - Ápr. 7. (2011)

Válogatta: Fábián Gábor

Tunable Kondo effect in graphene with defects

Jian-Hao Chen, Liang Li, William G. Cullen, Ellen D. Williams & Michael S. Fuhrer

Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer and a large number of electronic phenomena have been demonstrated in graphene, from the fractional quantum Hall effect to superconductivity. However, the coupling of conduction electrons to local magnetic moments, a central problem of condensed-matter physics, has not been realized in graphene, and, given carbon’s lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported, with explanations ranging from lattice defects to edge structures to negative curvature regions of the graphene sheet. Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG), induced by proton irradiation or native to grain boundaries, can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene are local moments which interact strongly with the conduction electrons through the Kondo effect, providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature (T_K) is tunable with carrier density from 30 to 90 K; the high (T_K) is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material.

Published in Nature Physics: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1962.html

Nanoscale Joule heating, Peltier cooling and current crowding at graphene–metal contacts

Kyle L. Grosse, Myung-Ho Bae, Feifei Lian, Eric Pop & William P. King

The performance and scaling of graphene-based electronic is limited by the quality of contacts between the graphene and metal electrodes. However, the nature of graphene–metal contacts remains incompletely understood. Here, we use atomic force microscopy to measure the temperature distributions at the contacts of working graphene transistors with a spatial resolution of ~10 nm (refs 5, 6, 7, 8), allowing us to identify the presence of Joule heating current crowding and thermoelectric heating and cooling. Comparison with simulation enables extraction of the contact resistivity (150–200 Ω µm²) and transfer length (0.2–0.5 µm) in our devices; these generally limit performance and must be minimized. Our data indicate that thermoelectric effects account for up to one-third of the contact temperature changes, and that current crowding accounts for most of the remainder. Modelling predicts that the role of current crowding will diminish and the role of thermoelectric effects will increase as contacts improve.

Published in Nature Nanotechnology: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.39.html

Heavily Doped Semiconductor Nanocrystal Quantum Dots

David Mocatta, Guy Cohen, Jonathan Schattner, Oded Millo, Eran Rabani, Uri Banin

Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.

Published in Science: http://www.sciencemag.org/content/332/6025/77.full

Resonant Vibrations, Peak Broadening, and Noise in Single Molecule Contacts: The Nature of the First Conductance Peak

Daniel Secker, Stefan Wagner, Stefan Ballmann, Rainer Härtle, Michael Thoss, and Heiko B. Weber

We carry out experiments on single-molecule junctions at low temperatures, using the mechanically controlled break junction technique. Analyzing the results obtained with various molecules, the nature of the first peak in the differential conductance spectra is elucidated. We observe an electronic transition with a vibronic fine structure, if the first peak occurs at small voltages. This regime can accurately be described by the resonant tunneling model. At higher voltages, additional smearing is observed and no fine structure can be resolved. A detailed analysis of the noise signal indicates that the onset of current is associated with strong fluctuations as a precursor of current flow. The data indicate that a complex fluctuation-driven transport mechanism takes over in this regime.

Published in Phys. Rev. Lett. 106: http://link.aps.org/doi/10.1103/PhysRevLett.106.136807

Graphene Valley Filter Using a Line Defect

D. Gunlycke and C. T. White

With its two degenerate valleys at the Fermi level, the band structure of graphene provides the opportunity to develop unconventional electronic applications. Herein, we show that electron and hole quasiparticles in graphene can be filtered according to which valley they occupy without the need to introduce confinement. The proposed valley filter is based on scattering off a recently observed line defect in graphene. Quantum transport calculations show that the line defect is semitransparent and that quasiparticles arriving at the line defect with a high angle of incidence are transmitted with a valley polarization near 100%.

Published in Phys. Rev. Lett. 106 (Editor's suggestion): http://link.aps.org/doi/10.1103/PhysRevLett.106.136806

Imaging charge density fluctuations in graphene using Coulomb blockade spectroscopy

A. Deshpande, W. Bao, Z. Zhao, C. N. Lau, and B. J. LeRoy

Using scanning tunneling microscopy (STM), 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 potential on the graphene. We observe a series of electron and hole doped puddles with a characteristic length scale of ∼20 nm. Theoretical calculations for the correlation length of the puddles based on the number of impurities are in agreement with our measurements.

Published in Phys. Rev. B 83 (Editor's suggestion):http://link.aps.org/doi/10.1103/PhysRevB.83.155409

Electrical transport through a mechanically gated molecular wire

C. Toher, R. Temirov, A. Greuling, F. Pump, M. Kaczmarski, G. Cuniberti, M. Rohlfing, and F. S. Tautz

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a predefined 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 nonequilibrium Green's function (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.

Published in Phys. Rev. B 83 (Editor's suggestion): http://link.aps.org/doi/10.1103/PhysRevB.83.155402

Improving transition voltage spectroscopy of molecular junctions

Troels Markussen, Jingzhe Chen, and Kristian S. Thygesen

Transition voltage spectroscopy (TVS) is a promising spectroscopic tool for molecular junctions. The principles in TVS is to find the minimum on a Fowler-Nordheim plot where ln(I/V²) is plotted against 1/V and relate the voltage at the minimum Vmin to the closest molecular level. Importantly, Vmin is approximately half the voltage required to see a peak in the dI/dV curve. Information about the molecular level position can thus be obtained at relatively low voltages. In this work we show that the molecular level position can be determined at even lower voltages, V_min^(α), by finding the minimum of ln(I/V^α) with α<2. On the basis of a simple Lorentzian transmission model we analyze theoretical ab initio as well as experimental I-V curves and show that the voltage required to determine the molecular levels can be reduced by ~30% as compared to conventional TVS. As for conventional TVS, the symmetry/asymmetry of the molecular junction needs to be taken into account in order to gain quantitative information. We show that the degree of asymmetry may be estimated from a plot of V_min(α) vs α.

Published in Phys. Rev. B 83: http://link.aps.org/doi/10.1103/PhysRevB.83.155407

Room-Temperature Charge Stability Modulated by Quantum Effects in a Nanoscale Silicon Island

S. J. Shin, J. J. Lee, H. J. Kang, J. B. Choi, S.-R. Eric Yang, Y. Takahashi, and D. G. Hasko

We report on transport measurement performed on a room-temperature-operating ultrasmall Coulomb blockade devices with a silicon island of sub5 nm. The charge stability at 300K exhibits a substantial change in slopes and diagonal size of each successive Coulomb diamond, but remarkably its main feature persists even at low temperature down to 5.3K except for additional Coulomb peak splitting. This key feature of charge stability with additional fine structures of Coulomb peaks are successfully modeled by including the interplay between Coulomb interaction, valley splitting, and strong quantum confinement, which leads to several low-energy many-body excited states for each dot occupancy. These excited states become enhanced in the sub5 nm ultrasmall scale and persist even at 300K in the form of cluster, leading to the substantial modulation of charge stability.

Published in Nano Letters: http://pubs.acs.org/doi/abs/10.1021/nl1044692

Super-Elastic Graphene Ripples for Flexible Strain Sensors

Yi Wang, Rong Yang, Zhiwen Shi, Lianchang Zhang, Dongxia Shi, Enge Wang, and Guangyu Zhang

In this study, we report a buckling approach for graphene and graphene ribbons on stretchable elastomeric substrates. Stretched polydimethylsiloxane (PDMS) films with different prestrains were used to receive the transferred graphene, and nanoscale periodical buckling of graphene was spontaneously formed after strain release. The morphology and periodicity of the as-formed graphene ripples are dependent strongly on their original shapes and substrates’ prestrains. Regular periodicity of the ripples preferred to form for narrow graphene ribbons, and both the amplitude and periodicity are reduced with the increase of prestrain on PDMS. The graphene ripples have the ability to afford large strain deformation, thus making it ideal for flexible electronic applications. It was demonstrated that both graphene ribbon and nanographene film ripples could be used for strain sensors, and their resistance changes upon different strains were studied. This simple and controllable process of buckled graphene provides a feasible fabrication for graphene flexible electronic devices and strain sensors due to its novel mechanical and electrical properties.

Published in ACS Nano: http://pubs.acs.org/doi/abs/10.1021/nn103523t

Integer Quantum Hall Eﬀect in Trilayer Graphene

A. Kumar, W. Escoﬃer, J.M. Poumirol, C. Faugeras, D. P. Arovas, M. M. Fogler, F. Guinea, S. Roche, M. Goiran and B. Raquet

The Integer Quantum Hall Eﬀect (IQHE) is a distinctive phase of two-dimensional electronic systems subjected to a perpendicular magnetic ﬁeld. Thus far, the IQHE has been observed in semiconductor heterostructures and in mono- and bi-layer graphene. Here we report on the IQHE in a new system: trilayer graphene. Experimental data are compared with self-consistent Hartree calculations of the Landau levels for the gated trilayer. The plateau structure in the Hall resistivity determines the stacking order (ABA versus ABC). We ﬁnd that the IQHE in ABC trilayer graphene is similar to that in the monolayer, except for the absence of a plateau at ﬁlling factor ν = 2. At very low ﬁlling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder.

Preprint online at arXiv: http://xxx.lanl.gov/abs/1104.1020

Quantum Hall effect and Landau level crossing of Dirac fermions in trilayer graphene

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

We investigate electronic transport in high mobility (\textgreater 100,000 cm$^2$/V$\cdot$s) trilayer graphene devices on hexagonal boron nitride, which enables the observation of Shubnikov-de Haas oscillations and an unconventional quantum Hall effect. The massless and massive characters of the TLG subbands lead to a set of Landau level crossings, whose magnetic field and filling factor coordinates enable the direct determination of the Slonczewski-Weiss-McClure (SWMcC) parameters used to describe the peculiar electronic structure of trilayer graphene. Moreover, at high magnetic fields, the degenerate crossing points split into manifolds indicating the existence of broken-symmetry quantum Hall states.

Preprint online at arXiv: http://xxx.lanl.gov/abs/1104.0438v1

Stacking-Dependent Band Gap and Quantum Transport in Trilayer Graphene

W. Bao, L. Jing, Y. Lee, J. Velasco Jr., P. Kratz, D. Tran, B. Standley, M. Aykol, S. B. Cronin, D. Smirnov, M. Koshino, E. McCann, M. Bockrath, C.N. Lau

In a multi-layer electronic system, stacking order provides a rarely-explored degree of freedom for tuning its electronic properties. Here we demonstrate the dramatically different transport properties in trilayer graphene (TLG) with different stacking orders. At the Dirac point, ABA-stacked TLG remains metallic while the ABC counterpart becomes insulating. The latter exhibits a gap-like dI/dV characteristics at low temperature and thermally activated conduction at higher temperatures, indicating an intrinsic gap ~6 meV. In magnetic fields, in addition to an insulating state at filling factor {\nu}=0, ABC TLG exhibits quantum Hall plateaus at {\nu}=-30, \pm 18, \pm 9, each of which splits into 3 branches at higher fields. Such splittings are signatures of the Lifshitz transition induced by trigonal warping, found only in ABC TLG, and in semi-quantitative agreement with theory. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications.

Preprint online at arXiv: http://xxx.lanl.gov/abs/1103.6088v1

Márc. 25. - Márc 31. (2011)

Válogatta: Piszter Gábor

Graphene Epitaxy by Chemical Vapor Deposition on SiC

W. Strupinski, K. Grodecki, A. Wysmolek, R. Stepniewski, T. Szkopek, P. E. Gaskell, A. Grüneis, D. Haberer, R. Bozek, J. Krupka, and J. M. Baranowski

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of 1800 cm²/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.

http://pubs.acs.org/doi/abs/10.1021/nl200390e

Low-Temperature Growth of Graphene by Chemical Vapor Deposition Using Solid and Liquid Carbon Sources

Zhancheng Li, Ping Wu, Chenxi Wang, Xiaodong Fan, Wenhua Zhang, Xiaofang Zhai, Changgan Zeng, Zhenyu Li, Jinlong Yang, and Jianguo Hou

Graphene has attracted a lot of research interest owing to its exotic properties and a wide spectrum of potential applications. Chemical vapor deposition (CVD) from gaseous hydrocarbon sources has shown great promises for large-scale graphene growth. However, high growth temperature, typically 1000 °C, is required for such growth. Here we demonstrate a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid and liquid hydrocarbon feedstocks. For solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down to 400 °C. When benzene is used as the hydrocarbon source, monolayer graphene flakes with excellent quality are achieved at a growth temperature as low as 300 °C. The successful low-temperature growth can be qualitatively understood from the first principles calculations. Our work might pave a way to an undemanding route for economical and convenient graphene growth.

http://pubs.acs.org/doi/abs/10.1021/nn200854p

High Throughput Nanofabrication of Silicon Nanowire and Carbon Nanotube Tips on AFM Probes by Stencil-Deposited Catalysts

Daniel S. Engstrom, Veronica Savu, Xueni Zhu, Ian Y. Y. Bu, William I. Milne, Juergen Brugger, and Peter Boggild

A new and versatile technique for the wafer scale nanofabrication of silicon nanowire (SiNW) and multiwalled carbon nanotube (MWNT) tips on atomic force microscope (AFM) probes is presented. Catalyst material for the SiNW and MWNT growth was deposited on prefabricated AFM probes using aligned wafer scale nanostencil lithography. Individual vertical SiNWs were grown epitaxially by a catalytic vapor−liquid−solid (VLS) process and MWNTs were grown by a plasma-enhanced chemical vapor (PECVD) process on the AFM probes. The AFM probes were tested for imaging micrometers-deep trenches, where they demonstrated a significantly better performance than commercial high aspect ratio tips. Our method demonstrates a reliable and cost-efficient route toward wafer scale manufacturing of SiNW and MWNT AFM probes.

http://pubs.acs.org/doi/abs/10.1021/nl104384b

Diffusion and drift of graphene flake on graphite surface

Irina V. Lebedeva, Andrey A. Knizhnik, Andrey M. Popov, Olga V. Ershova, Yurii E. Lozovik, Boris V. Potapkin

Diffusion and drift of a graphene flake on a graphite surface are analyzed. A potential energy relief of the graphene flake is computed using ab initio and empirical calculations. Based on the analysis of this relief, different mechanisms of diffusion and drift of the graphene flake on the graphite surface are considered. A new mechanism of diffusion and drift of the flake is proposed. According to the proposed mechanism, rotational transition of the flake from commensurate to incommensurate state takes place with subsequent simultaneous rotation and translational motion until a commensurate state is reached again, and so on. Analytic expressions for the diffusion coefficient and mobility of the flake corresponding to different mechanisms are derived in wide ranges of temperatures and sizes of the flake. The molecular dynamics simulations and estimates based on ab initio and empirical calculations demonstrate that the proposed mechanism can be dominant under certain conditions. The influence of structural defects on the diffusion of the flake is examined on the basis of calculations of the potential energy relief and molecular dynamics simulations. The methods of control over the diffusion and drift of graphene components in nanoelectromechanical systems are discussed. The possibility to experimentally determine the barriers to relative motion of graphene layers based on the study of diffusion of a graphene flake is considered. The results obtained can be also applied to polycyclic aromatic molecules on graphene and should be qualitatively valid for a set of commensurate adsorbate-adsorbent systems.

http://arxiv.org/abs/1103.5385

Manipulation of Electron Orbitals in Hard-Wall InAs/InP Nanowire Quantum Dots

Stefano Roddaro, Andrea Pescaglini, Daniele Ercolani, Lucia Sorba, and Fabio Beltram

We present a novel technique for the manipulation of the energy spectrum of hard-wall InAs/InP nanowire quantum dots. By using two local gate electrodes, we induce a strong transverse electric field in the dot and demonstrate the controlled modification of its electronic orbitals. Our approach allows us to dramatically enhance the single-particle energy spacing between the first two quantum levels in the dot and thus to increment the working temperature of our InAs/InP single-electron transistors. Our devices display a very robust modulation of the conductance even at liquid nitrogen temperature, while allowing an ultimate control of the electron filling down to the last free carrier. Potential further applications of the technique to time-resolved spin manipulation are also discussed.

http://pubs.acs.org/doi/abs/10.1021/nl200209m

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

A. Luican, Guohong Li, A. Reina, J. Kong, R. R. Nair, K. S. Novoselov, A. K. Geim, and E. Y. Andrei

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ∼3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

http://link.aps.org/doi/10.1103/PhysRevLett.106.126802

Electronic Sensitivity of Carbon Nanotubes to Internal Water Wetting

Di Cao, Pei Pang, Jin He, Tao Luo, Jae Hyun Park, Predrag Krstic, Colin Nuckolls, Jinyao Tang, and Stuart Lindsay

We have constructed devices in which the interior of a single-walled carbon nanotube (SWCNT) field-effect transistor acts as a nanofluidic channel that connects two fluid reservoirs, permitting measurement of the electronic properties of the SWCNT as it is wetted by an analyte. Wetting of the inside of the SWCNT by water turns the transistor on, while wetting of the outside has little effect. These observations are consistent with theoretical simulations that show that internal water both generates a large dipole electric field, causing charge polarization of the tube and metal electrodes, and shifts the valence band of the SWCNT, while external water has little effect. This finding may provide a new method to investigate water behavior at nanoscale. This also opens a new avenue for building sensors in which the SWCNT simultaneously functions as a concentrator, nanopore, and extremely sensitive electronic detector, exploiting the enhanced sensitivity of the interior surface.

http://pubs.acs.org/doi/abs/10.1021/nn200251z

Li-Air Rechargeable Battery Based on Metal-free Graphene Nanosheet Catalysts

Eunjoo Yoo and Haoshen Zhou

Metal-free graphene nanosheets (GNSs) were examined for use as air electrodes in a Li−air battery with a hybrid electrolyte. At 0.5 mA cm⁻¹, the GNSs showed a high discharge voltage that was near that of the 20 wt % Pt/carbon black. This was ascribed to the presence of sp³ bonding associated with edge and defect sites in GNSs. Moreover, heat-treated GNSs not only provided a similar catalytic activity in reducing oxygen in the air, but also showed a much more-stable cycling performance than GNSs when used in a rechargeable Li-air battery. This improvement resulted from removal of adsorbed functional groups and from crystallization of the GNS surface into a graphitic structure on heat treatment.

http://pubs.acs.org/doi/abs/10.1021/nn200084u

A stretchable carbon nanotube strain sensor for human-motion detection

Takeo Yamada, Yuhei Hayamizu, Yuki Yamamoto, Yoshiki Yomogida, Ali Izadi-Najafabadi, Don N. Futaba and Kenji Hata

Devices made from stretchable electronic materials could be incorporated into clothing or attached directly to the body. Such materials have typically been prepared by engineering conventional rigid materials such as silicon, rather than by developing new materials. Here, we report a class of wearable and stretchable devices fabricated from thin films of aligned single-walled carbon nanotubes. When stretched, the nanotube films fracture into gaps and islands, and bundles bridging the gaps. This mechanism allows the films to act as strain sensors capable of measuring strains up to 280% (50 times more than conventional metal strain gauges), with high durability, fast response and low creep. We assembled the carbonnanotube sensors on stockings, bandages and gloves to fabricate devices that can detect different types of human motion, including movement, typing, breathing and speech.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.36.html

Márc. 18. - Márc 24. (2011)

Válogatta: Márton Attila

Dynamic Control of Chiral Space in a Catalytic Asymmetric Reaction Using a Molecular Motor

Jiaobing Wang, Ben L. Feringa

Enzymes and synthetic chiral catalysts have found widespread application to produce single enantiomers, but in situ switching of the chiral preference of a catalytic system is very difficult to achieve. Here, we report on a light-driven molecular motor with integrated catalytic functions in which the stepwise change in configuration during a 360° unidirectional rotary cycle governs the catalyst performance both with respect to activity and absolute stereocontrol in an asymmetric transformation. During one full rotary cycle, catalysts are formed that provide either racemic (R,S) or preferentially the R or the S enantiomer of the chiral product of a conjugate addition reaction. This catalytic system demonstrates how different molecular tasks can be performed in a sequential manner, with the sequence controlled by the directionality of a rotary cycle.

http://www.sciencemag.org/content/331/6023/1429.abstract

Transforming C60 molecules into graphene quantum dots

Jiong Lu, Pei Shan Emmeline Yeo, Chee Kwan Gan, Ping Wu Kian Ping Loh Nature Nanotechnology (2011) doi:10.1038/nnano.2011.30 Published online 20 March 2011

The fragmentation of fullerenes using ions, surface collisions or thermal effects is a complex process that typically leads to the formation of small carbon clusters of variable size. Here, we show that geometrically well-defined graphene quantum dots can be synthesized on a ruthenium surface using C60molecules as a precursor. Scanning tunnelling microscopy imaging, supported by density functional theory calculations, suggests that the structures are formed through the ruthenium-catalysed cage-opening of C60. In this process, the strong C60–Ru interaction induces the formation of surface vacancies in the Ru single crystal and a subsequent embedding of C60 molecules in the surface. The fragmentation of the embedded molecules at elevated temperatures then produces carbon clusters that undergo diffusion and aggregation to form graphene quantum dots. The equilibrium shape of the graphene can be tailored by optimizing the annealing temperature and the density of the carbon clusters.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.30.html

Self-Healing of Fractured GaAs Nanowires

Yanbo Wang, Hannah J. Joyce, Qiang Gao, Xiaozhou Liao, H. Hoe Tan, Jin Zou, Simon P. Ringer, Zhiwei Shan, and Chennupati Jagadish Nano Lett., Article ASAP Publication Date (Web): March 18, 2011

In-situ deformation experiments were carried out in a transmission electron microscope to investigate the structural response of single crystal GaAs nanowires (NWs) under compression. A repeatable self-healing process was discovered in which a partially fractured GaAs NW restored its original single crystal structure immediately after an external compressive force was removed. Possible mechanisms of the self-healing process are discussed.

http://pubs.acs.org/doi/abs/10.1021/nl104330h

The Number of Transmission Channels Through a Single-Molecule Junction

Justin P. Bergfield†, Joshua D. Barr, and Charles A. Stafford ACS Nano, Article ASAP Publication Date (Web): March 21, 2011

We calculate transmission eigenvalue distributions for Pt−benzene−Pt and Pt−butadiene−Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting π-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level.

http://pubs.acs.org/doi/abs/10.1021/nn1030753

Air-Stable Conversion of Separated Carbon Nanotube Thin-Film Transistors from p-Type to n-Type Using Atomic Layer Deposition of High-κ Oxide and Its Application in CMOS Logic Circuits

Jialu Zhang, Chuan Wang, Yue Fu, Yuchi Che, and Chongwu Zhou, ACS Nano, Article ASAP Publication Date (Web): March 18, 2011

Due to extraordinary electrical properties, preseparated, high purity semiconducting carbon nanotubes hold great potential for thin-film transistors (TFTs) and integrated circuit applications. One of the main challenges it still faces is the fabrication of air-stable n-type nanotube TFTs with industry-compatible techniques. Here in this paper, we report a novel and highly reliable method of converting the as-made p-type TFTs using preseparated semiconducting nanotubes into air-stable n-type transistors by adding a high-κ oxide passivation layer using atomic layer deposition (ALD). The n-type devices exhibit symmetric electrical performance compared with the p-type devices in terms of on-current, on/off ratio, and device mobility. Various factors affecting the conversion process, including ALD temperature, metal contact material, and channel length, have also been systematically studied by a series of designed experiments. A complementary metal−oxide−semiconductor (CMOS) inverter with rail-to-rail output, symmetric input/output behavior, and large noise margin has been further demonstrated. The excellent performance gives us the feasibility of cascading multiple stages of logic blocks and larger scale integration. Our approach can serve as the critical foundation for future nanotube-based thin-film macroelectronics.

http://pubs.acs.org/doi/abs/10.1021/nn2004298

Coulomb blockade and hopping conduction in graphene quantum dots array

Daeha Joung, Lei Zhai1 and Saiful I. Khondaker Published 21 March 2011

We show that the low-temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two-dimensional array of graphene quantum dots (GQDs). Below 15 K, a total suppression of current due to Coulomb blockade through a GQD array was observed. Temperature-dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2–10 meV corresponding to GQD sizes of 5–8 nm, while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural- and size-induced disorder.

http://prb.aps.org/abstract/PRB/v83/i11/e115323

Spin-orbit interaction in curved graphene ribbons

D. Gosálbez-Martínez, J. J. Palacios and J. Fernández-Rossier ublished 16 March 2011

We study the electronic properties of electrons in flat and curved zigzag graphene nanoribbons using a tight-binding model within the Slater Koster approximation, including spin-orbit interaction. We find that a constant curvature across the ribbon dramatically enhances the action of the spin-orbit term, strongly influencing the spin orientation of the edge states: Whereas spins are normal to the surface in the case of flat ribbons, this is no longer the case for curved ribbons. This effect is very pronounced, the spins deviating from the normal to the ribbon, even for very small curvature and a realistic spin orbit coupling of carbon. We find that curvature results also in an effective second neighbor hopping that modifies the electronic properties of zigzag graphene ribbons. We discuss the implications of our findings in the spin Hall phase of curved graphene ribbons.

http://prb.aps.org/abstract/PRB/v83/i11/e115436

Márc. 11. - Márc 17. (2011)

Válogatta: Scherübl Zoltán

A Single-Molecule Potentiometer

Controlling electron transport through a single-molecule device is key to the realization of nanoscale electronic components. A design requirement for single molecule electrical devices is that the molecule must be both structurally and electrically connected to the metallic electrodes. Typically, the mechanical and electrical contacts are achieved by the same chemical moiety. In this study, we demonstrate that the structural role may be played by one group (for example, a sulfide) while the electrical role may be played by another (a conjugated chain of C═C π-bonds). We can specify the electrical conductance through the molecule by modulating to which particular site on the oligoene chain the electrode binds. The result is a device that functions as a potentiometer at the single-molecule level.

http://pubs.acs.org/doi/abs/10.1021/nl104411f

A Fully Tunable Single-Walled Carbon Nanotube Diode

We demonstrate a fully tunable diode structure utilizing a fully suspended single-walled carbon nanotube. The diode’s turn-on voltage under forward bias can be continuously tuned up to 4.3 V by controlling gate voltages, which is 6 times the nanotube band gap energy. Furthermore, the same device design can be configured into a backward diode by tuning the band-to-band tunneling current with gate voltages. A nanotube backward diode is demonstrated for the first time with nonlinearity exceeding the ideal diode. These results suggest that a tunable nanotube diode can be a unique building block for developing next generation programmable nanoelectronic logic and integrated circuits.

http://pubs.acs.org/doi/abs/10.1021/nl200371z

Self-Aligned U-Gate Carbon Nanotube Field-Effect Transistor with Extremely Small Parasitic Capacitance and Drain-Induced Barrier Lowering

A novel self-aligned U-gate structure for carbon nanotube (CNT) field-effect transistors (FETs) is introduced and shown to yield excellent dc properties and high reproducibility that are comparable with that of the best CNT FETs based on the previously developed self-aligned device structures. In particular the subthreshold swing of the U-gate FET is 75 mV/dec and the drain-induced barrier lowering is effectively zero, indicating that the electrostatic potential of the whole CNT channel is most efficiently controlled by the U-gate and that the CNT device is a well-behaved FET. Moreover the high-frequency response of the U-gate FET is investigated, and the parasitic capacitance of the device is measured and shown to be one magnitude smaller than that of the previously developed self-aligned device structures. Direct frequency domain measurements show that the U-gate CNT FETs can operate up to 800 MHz, which is also higher than previously reported values. The large improvement in the device high-frequency behavior is largely due to the replacement of the high-κ dielectric material between the source/drain and the gate by a vacant space with κ ≈ 1, and the significant reduction in the device parasitic capacitance renders the U-gate CNT FETs promising for rf applications.

http://pubs.acs.org/doi/abs/10.1021/nn102091h

Flexible high-performance carbon nanotube integrated circuits

Carbon nanotube thin-film transistors1 are expected to enable the fabrication of high-performance2, flexible3 and transparent4 devices using relatively simple techniques. However, as-grown nanotube networks usually contain both metallic and semiconducting nanotubes, which leads to a trade-off between charge-carrier mobility (which increases with greater metallic tube content) and on/off ratio (which decreases)5. Many approaches to separating metallic nanotubes from semiconducting nanotubes have been investigated6, 7, 8, 9, 10, 11, but most lead to contamination and shortening of the nanotubes, thus reducing performance. Here, we report the fabrication of high-performance thin-film transistors and integrated circuits on flexible and transparent substrates using floating-catalyst chemical vapour deposition followed by a simple gas-phase filtration and transfer process. The resulting nanotube network has a well-controlled density and a unique morphology, consisting of long (~10 µm) nanotubes connected by low-resistance Y-shaped junctions. The transistors simultaneously demonstrate a mobility of 35 cm2 V–1 s–1 and an on/off ratio of 6 × 106. We also demonstrate flexible integrated circuits, including a 21-stage ring oscillator and master–slave delay flip-flops that are capable of sequential logic. Our fabrication procedure should prove to be scalable, for example, by using high-throughput printing techniques.

http://www.nature.com/nnano/journal/v6/n3/pdf/nnano.2011.1.pdf

Comparison between charge and spin transport in few-layer graphene

Transport measurements on few-layer graphene (FLG) are important because they interpolate between the properties of single-layer graphene (SLG) as a true two-dimensional material and the three-dimensional bulk properties of graphite. In this article we present four-probe local charge transport and nonlocal spin-valve and spin-precession measurements on lateral spin field-effect transistors on FLG. We study systematically the charge- and spin-transport properties depending on the number of layers and the electrical back gating of the device. We explain the charge-transport measurements by taking the screening of scattering potentials into account and use the results to understand the spin data. The measured samples are between 3 and 20 layers thick, and we include in our analysis our earlier results of the measurements on SLG for comparison. In our room-temperature spin-transport measurements we manage to observe spin signals over distances up to 10 μm and measure enhanced spin-relaxation times with an increasing number of layers, reaching τs~500 ps as a maximum, about 4 times higher than in SLG. The increase of τs can result from the screening of scattering potentials due to additional intrinsic charge carriers in FLG. We calculate the density of states of FLG using a zone-folding scheme to determine the charge-diffusion coefficient DC from the square resistance RS. The resulting DC and the spin-diffusion coefficient DS show similar values and depend only weakly on the number of layers and gate-induced charge carriers. We discuss the implications of this on the identification of the spin-relaxation mechanism.

http://prb.aps.org/abstract/PRB/v83/i11/e115410

Hysteresis loops of magnetoconductance in graphene devices

We report very low-temperature magnetoconductance ΔG measurements on graphene devices with the magnetic field H applied parallel to the carbon sheet. The ΔG(H) signal depends on the gate voltage Vg and its sign is related to the universal conductance fluctuations. When the magnetic field is swept at fast rates, ΔG displays hysteresis loops evident for different sizes and at different transport regimes of the devices. We attribute this to the magnetization reversal of paramagnetic centers in the graphene layer, which might originate from defects in our devices.

http://prb.aps.org/abstract/PRB/v83/i12/e121401

Transport scattering time probed through rf admittance of a graphene capacitor

We have investigated electron dynamics in top gated graphene by measuring the gate admittance of a diffusive graphene capacitor in a broad frequency range as a function of carrier density. The density of states, conductivity, and diffusion constant are deduced from the low-frequency gate capacitance, its charging time, and their ratio. The admittance evolves from an rc-like to a skin-effect response at GHz frequency with a crossover given by the Thouless energy. The scattering time is found to be independent of energy in the 0- to 200-meV investigated range at room temperature. This is consistent with a random mass model for Dirac fermions.

http://prb.aps.org/abstract/PRB/v83/i12/e125408

Ultrathin body InAs tunneling field-effect transistors on Si substrates

An ultrathin body InAs tunneling field-effect transistor on Si substrate is demonstrated by using an epitaxial layer transfer technique. A postgrowth, zinc surface doping approach is used for the formation of a p+ source contact which minimizes lattice damage to the ultrathin body InAs compared to ion implantation. The transistor exhibits gated negative differential resistance behavior under forward bias, confirming the tunneling operation of the device. In this device architecture, the ON current is dominated by vertical band-to-band tunneling and is thereby less sensitive to the junction abruptness. The work presents a device and materials platform for exploring III–V tunnel transistors.

http://apl.aip.org/resource/1/applab/v98/i11/p113105_s1

Energy relaxation in graphene and its measurement with supercurrent

We study inelastic energy relaxation in graphene for low energies to find out how electrons scatter with acoustic phonons and other electrons. By coupling the graphene to superconductors, we create a strong dependence of the measured signal, i.e.,\ critical Josephson current, on the electron population on different energy states. Since the relative population of high- and low-energy states is determined by the inelastic scattering processes, the critical current becomes an effective probe for their strength. We argue that the electron-electron interaction is the dominant relaxation method and, in our model of two-dimensional electron-electron scattering, we find a scattering time $\tau_{e-e}=5... 13$ ps at T=500 mK, 1-2 orders of magnitude smaller than predicted by theory.

http://www.arxiv.org/abs/1103.3234

Márc. 4. - Márc 10. (2011)

Válogatta: Márton Attila

Evaluation of conduction eigenchannels of an adatom probed by an STM tip

Martyna Polok, Dmitry V. Fedorov, Alexei Bagrets, Peter Zahn, Ingrid Mertig

Ballistic conductance through a single atom adsorbed on a metallic surface and probed by a scanning tunneling microscope (STM) tip can be decomposed into eigenchannel contributions, which can be potentially obtained from shot noise measurements. Our density functional theory calculations provide evidence that transmission probabilities of these eigenchannels encode information on the modifications of the adatom's local density of states caused by its interaction with the STM tip. In the case of open shell atoms, this can be revealed in nonmonotonic behavior of the eigenchannel's transmissions as a function of the tip-adatom separation.

http://xxx.lanl.gov/abs/1103.1162

Length and temperature dependent crossover of charge transport across molecular junctions

Ya-Lin Lo, Shih-Jye Sun, Ying-Jer Kao

We study the electronic transport in a molecular junction in which each site is coupled to a local phonon bath using the non-equilibrium Green's function method. We observe the length period of the oscillatory conductance in odd-numbered chains depends strongly on the applied bias, and the oscillatory behavior is smeared out for the bias voltage near the phonon energy. In addition, a crossover from tunneling to thermally activated hopping transport as the length of the molecule increases is found for the phonon-free case. In the presence of electron-phonon interaction, hopping transport is dominant and a transition from the thermally suppressed to assisted conduction is observed.

http://xxx.lanl.gov/abs/1103.1708

Low-Power Switching of Phase-Change Materials with Carbon Nanotube Electrodes

Feng Xiong, Albert Liao, David Estrada and Eric Pop

Phase-change materials (PCMs) are promising candidates for nonvolatile data storage and reconfigurable electronics, but high programming currents have presented a challenge to realize low power operation. We controlled PCM bits with single-wall and small-diameter multi-wall carbon nanotubes. This configuration achieves programming currents as low as 0.5 μA (SET) and 5 μA (RESET), two orders of magnitude lower than state-of-the-art devices. Pulsed measurements enable memory switching with very low energy consumption. Analysis of over 100 devices finds that the programming voltage and energy are highly scalable, and could be below 1 V and single femtojoules per bit, respectively.

http://www.sciencemag.org/content/early/2011/03/09/science.1201938.abstract?sid=68753e44-805a-4ee7-b674-ad06e46249d3

Layer-by-Layer Removal of Graphene for Device Patterning

Ayrat Dimiev, Dmitry V. Kosynkin, Alexander Sinitskii, Alexander Slesarev, Zhengzong Sun, and James M. Tour

The patterning of graphene is useful in fabricating electronic devices, but existing methods do not allow control of the number of layers of graphene that are removed. We show that sputter-coating graphene and graphene-like materials with zinc and dissolving the latter with dilute acid removes one graphene layer and leaves the lower layers intact. The method works with the four different types of graphene and graphene-like materials: graphene oxide, chemically converted graphene, chemical vapor–deposited graphene, and micromechanically cleaved (“clear-tape”) graphene. On the basis of our data, the top graphene layer is damaged by the sputtering process, and the acid treatment removes the damaged layer of carbon. When used with predesigned zinc patterns, this method can be viewed as lithography that etches the sample with single-atomic-layer resolution.

http://www.sciencemag.org/content/331/6021/1168.abstract?sid=68753e44-805a-4ee7-b674-ad06e46249d3

Local Peeling of Graphene

Daniel Gunlycke and Paul E. Sheehan

“Perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away,” noted Antoine de Saint-Exupéry (1). He could have been writing about graphene sheets, just an atomic layer or two thick, which have properties much more interesting than those of the bulk. From the time when graphite would be rubbed on an insulating surface with the hope that one of the exfoliated flakes would be a single layer, graphene manufacture has progressed rapidly and is now routinely grown on or transferred onto many substrates, even up to sizes large enough for TV displays (2). Despite such advances, reproducible spatial control over the number of graphene layers has not been achieved. On page 1168 of this issue, Dimiev et al. describe a technique that overcomes this limitation and allows peeling of graphene layer by layer at predetermined locations of the surface (3).

http://www.sciencemag.org/content/331/6021/1146.summary?sid=68753e44-805a-4ee7-b674-ad06e46249d3

Ultrathin Planar Graphene Supercapacitors

Jung Joon Yoo, Kaushik Balakrishnan, Jingsong Huang, Vincent Meunier, Bobby G. Sumpter, Anchal Srivastava, Michelle Conway, Arava Leela Mohana Reddy, Jin Yu, Robert Vajtai, and Pulickel M. Ajayan

With the advent of atomically thin and flat layers of conducting materials such as graphene, new designs for thin film energy storage devices with good performance have become possible. Here, we report an “in-plane” fabrication approach for ultrathin supercapacitors based on electrodes comprised of pristine graphene and multilayer reduced graphene oxide. The in-plane design is straightforward to implement and exploits efficiently the surface of each graphene layer for energy storage. The open architecture and the effect of graphene edges enable even the thinnest of devices, made from as grown 1−2 graphene layers, to reach specific capacities up to 80 μFcm−2, while much higher (394 μFcm−2) specific capacities are observed multilayer reduced graphene oxide electrodes. The performances of devices with pristine as well as thicker graphene-based structures are examined using a combination of experiments and model calculations. The demonstrated all solid-state supercapacitors provide a prototype for a broad range of thin-film based energy storage devices.

http://pubs.acs.org/doi/abs/10.1021/nl200225j

Interedge magnetic coupling in transition-metal terminated graphene nanoribbons

Yan Wang and 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 Å. 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 independently of the ribbon width and Ni-AGNRs are found to be nonmagnetic.

http://prb.aps.org/abstract/PRB/v83/i11/e113402

Analysis of current and shot noise correlations in a double quantum dot interferometer with interdot spin interactions

Marko Zivkovic, Brandon W. Langley, Ivana Djuric, and Chris P. Search

We examine an electron Aharonov-Bohm (AB) interferometer with individual quantum dots connected in parallel to macroscopic leads. Here, we focus on the effect that both interdot spin-spin exchange interactions and intradot spin flips have on the current- and frequency-dependent current shot noise. By appropriate control of AB magnetic flux, interdot Coulomb repulsion, intradot spin flips, and interdot spin-spin coupling, the probability amplitudes for the different paths of the interferometer can be controlled, leading to broad tunability of both the shape and the contrast of interference fringes in the current. We also show that in the shot noise at finite frequencies corresponding to the spin-spin interaction energies, the noise shows a pronounced super-Poissonian and sub-Poissonian structure. AB flux, which is not an integer multiple of 2π, dramatically suppresses the correlations in the shot noise.

http://prb.aps.org/abstract/PRB/v83/i11/e115304

Chiral spin textures of strongly interacting particles in quantum dots

Catherine J. Stevenson and Jordan Kyriakidis

We probe for statistical and Coulomb-induced spin textures among the low-lying states of repulsively interacting particles confined to potentials that are both rotationally and time-reversal invariant. In particular, we focus on two-dimensional quantum dots and employ configuration-interaction techniques to directly compute the correlated many-body eigenstates of the system. We produce spatial maps of the single-particle charge and spin density and verify the annular structure of the charge density and the rotational invariance of the spin field. We further compute two-point spin correlations to determine the correlated structure of a single component of the spin vector field. In addition, we compute three-point spin-correlation functions to uncover chiral structures. We present evidence for both chiral and quasitopological spin textures within energetically degenerate subspaces in the three- and four-particle systems.

http://prb.aps.org/abstract/PRB/v83/i11/e115306

Comparison between charge and spin transport in few-layer graphene

T. Maassen, F. K. Dejene, M. H. D. Guimarães, C. Józsa, and B. J. van Wees

Transport measurements on few-layer graphene (FLG) are important because they interpolate between the properties of single-layer graphene (SLG) as a true two-dimensional material and the three-dimensional bulk properties of graphite. In this article we present four-probe local charge transport and nonlocal spin-valve and spin-precession measurements on lateral spin field-effect transistors on FLG. We study systematically the charge- and spin-transport properties depending on the number of layers and the electrical back gating of the device. We explain the charge-transport measurements by taking the screening of scattering potentials into account and use the results to understand the spin data. The measured samples are between 3 and 20 layers thick, and we include in our analysis our earlier results of the measurements on SLG for comparison. In our room-temperature spin-transport measurements we manage to observe spin signals over distances up to 10 μm and measure enhanced spin-relaxation times with an increasing number of layers, reaching τs~500 ps as a maximum, about 4 times higher than in SLG. The increase of τs can result from the screening of scattering potentials due to additional intrinsic charge carriers in FLG. We calculate the density of states of FLG using a zone-folding scheme to determine the charge-diffusion coefficient DC from the square resistance RS. The resulting DC and the spin-diffusion coefficient DS show similar values and depend only weakly on the number of layers and gate-induced charge carriers. We discuss the implications of this on the identification of the spin-relaxation mechanism.

http://prb.aps.org/abstract/PRB/v83/i11/e115410

Kondo effects and shot noise enhancement in a laterally coupled double quantum dot

Toshihiro Kubo, Yasuhiro Tokura, and Seigo Tarucha

Spin and orbital Kondo effects and the related shot noise for a laterally coupled double quantum dot are studied taking account of coherent indirect coupling via a reservoir. We calculate the linear conductance and shot noise for various charge states to distinguish between the spin and the orbital Kondo effects. We find that a novel antiferromagnetic exchange coupling can be generated by the coherent indirect coupling, and it works to suppress the spin Kondo effect when each quantum dot holds just one electron. We also show that we can capture the feature of the pseudospin Kondo effect from the shot noise measurement.

http://prb.aps.org/abstract/PRB/v83/i11/e115310

Electron-electron interaction and electron-hole asymmetry in bilayer graphene

K. Zou, X. Hong, J. Zhu

We report precision measurements of the effective mass m* in high-quality bilayer graphene using the temperature dependence of the Shubnikov-de Haas oscillations. In the density range of 0.7 x 10^12/cm^2 < n < 4.1 x 10^12 /cm^2, both the hole mass m*_h and the electron mass m*_e increase with increasing density, demonstrating the hyperbolic nature of the bands. The hole mass m*_h is approximately 20-30% larger than the electron mass m*_e. Tight-binding calculations provide a good description of the electron-hole asymmetry and yield an accurate measure of the inter-layer hopping parameter v_4 = 0.063. Both m*_h and m*_e are substantially suppressed compared to single-particle values, providing clear and unprecedented evidence for the strong renormalization effect of electron-electron interaction in the band structure of bilayer graphene.

http://xxx.lanl.gov/abs/1103.1663

még érdekes cikkek:

Molecular Motors, Beauty in Complexity :

http://www.sciencemag.org/content/331/6021/1143.summary?sid=68753e44-805a-4ee7-b674-ad06e46249d3

Carbon nanotubes: The ins and outs of DNA :

http://www.nature.com/nnano/reshigh/2011/0311/full/nnano.2011.33.html

Higher-Energy Composite Fermion Levels in the Fractional Quantum Hall Effect :

http://prl.aps.org/abstract/PRL/v106/i9/e096803

Observation of Intrinsic Inverse Spin Hall Effect :

http://prl.aps.org/abstract/PRL/v106/i10/e107205

Feb 24. - Márc. 3. (2011)

Válogatta: Pósa László

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

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

The bias dependence of the torque that a spin-polarized current exerts on ferromagnetic elements is important for understanding fundamental spin physics in magnetic devices and for applications. Several experimental techniques have been introduced in recent years in attempts to measure spin-transfer torque in magnetic tunnel junctions. However, these techniques have provided only indirect measures of the torque and their results regarding bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in magnetic tunnel junctions 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 artefacts 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.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1928.html

Observation of supercurrent in PbIn-graphene-PbIn Josephson junction

Dongchan Jeong, Jae-Hyun Choi, Gil-Ho Lee, Sanghyun Jo, Yong-Joo Doh, Hu-Jong Lee

Superconductor-graphene-superconductor (SGS) junction provides a unique platform to study relativistic electrodynamics of Dirac fermions in graphene combined with proximity-induced superconductivity. We report the observation of the Josephson effect in proximity-coupled superconducting junctions of graphene in contact with Pb1-xInx (x=0.07) electrodes for temperatures as high as T = 4.8 K, with a large value of IcRN (~255 μ V). This demonstrates that Pb1-xInx SGS junction would facilitate the development of the superconducting quantum information devices and superconductor-enhanced phase-coherent transport of graphene.

http://prb.aps.org/abstract/PRB/v83/i9/e094503

Superconducting Junction of a Single-Crystalline Au Nanowire for an Ideal Josephson Device

Minkyung Jung, Hyunho Noh, Yong-Joo Doh, Woon Song, Yonuk Chong, Mahn-Soo Choi, Youngdong Yoo, Kwanyong Seo, Nam Kim, Byung-Chill Woo, Bongsoo Kim, and Jinhee Kim

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. The current−voltage characteristic curve shows a clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The subgap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport.

http://pubs.acs.org/doi/abs/10.1021/nn1035679

Second-Order Overtone and Combination Raman Modes of Graphene Layers in the Range of 1690−2150 cm−1

Chunxiao Cong, Ting Yu, Riichiro Saito, Gene F. Dresselhaus, and Mildred S. Dresselhaus

Though graphene has been intensively studied by Raman spectroscopy, in this letter, we report a study of the second-order overtone and combination Raman modes in a mostly unexplored frequency range of 1690−2150 cm−1 in nonsuspended commensurate (AB-stacked), incommensurate (folded) and suspended graphene layers. On the basis of the double resonance theory, four dominant modes in this range have been assigned to (i) the second order out-of-plane transverse mode (2oTO or M band), (ii) the combinational modes of in-plane transverse acoustic mode and longitudinal optical mode (iTA+LO), (iii) in-plane transverse optical mode and longitudinal acoustic mode (iTO+LA), and (iv) longitudinal optical mode and longitudinal acoustic mode (LO+LA). Differing from AB-stacked bilayer graphene or few layer graphene, single layer graphene shows the disappearance of the M band. Systematic analysis reveals that interlayer interaction is essential for the presence (or absence) of the M band, whereas the substrate has no effect on the presence (or absence) of the M band. Dispersive behaviors of these “new” Raman modes in graphene have been probed by laser excitation energy-dependent Raman spectroscopy. It is found that the appearance of the M band strictly depends on the AB stacking, which could be used as a fingerprint for AB-stacked bilayer graphene. This work expands upon the unique and powerful abilities of Raman spectroscopy to study graphene and provides another effective way to probe phonon dispersion, electron−phonon coupling, and to exploit the electronic band structure of graphene layers.

http://pubs.acs.org/doi/abs/10.1021/nn200010m

Mechanics and Chemistry: Single Molecule Bond Rupture Forces Correlate with Molecular Backbone Structure

Michael Frei, Sriharsha V. Aradhya, Max Koentopp, Mark S. Hybertsen, and L. Venkataraman

We simultaneously measure conductance and force across nanoscale junctions. A new, two-dimensional histogram technique is introduced to statistically extract bond rupture forces from a large data set of individual junction elongation traces. For the case of Au point contacts, we find a rupture force of 1.4 ± 0.2 nN, which is in good agreement with previous measurements. We then study systematic trends for single gold metal−molecule−metal junctions for a series of molecules terminated with amine and pyridine linkers. For all molecules studied, single molecule junctions rupture at the Au−N bond. Selective binding of the linker group allows us to correlate the N−Au bond-rupture force to the molecular backbone. We find that the rupture force ranges from 0.8 nN for 4,4′ bipyridine to 0.5 nN in 1,4 diaminobenzene. These experimental results are in excellent quantitative agreement with density functional theory based adiabatic molecular junction elongation and rupture calculations.

http://pubs.acs.org/doi/abs/10.1021/nl1042903

Alignment of Colloidal Graphene Quantum Dots on Polar Surfaces

Irma P. Hamilton, Binsong Li, Xin Yan, and Liang-shi Li

Controlling the orientation of nanostructures with anisotropic shapes is essential for taking advantage of their anisotropic electrical, optical, and transport properties in electro-optical devices. For large-area alignment of nanocrystals, so far orientations are mostly induced and controlled by external physical parameters, such as applied fields or changes in concentration. Herein we report on assemblies of colloidal graphene quantum dots, a new type of disk-shaped nanostructures, on polar surfaces and the control of their orientations. We show that the orientations of the graphene quantum dots can be determined, either in- or out-of-plane with the substrate, by chemical functionalization that introduces orientation-dependent interactions between the quantum dots and the surfaces.

http://pubs.acs.org/doi/abs/10.1021/nl200298c

Graphene-based Spin Caloritronics

Minggang Zeng, Yuanping Feng, and Gengchiau Liang

hermally induced spin transport in magnetized zigzag graphene nanoribbons (M-ZGNRs) is explored using first-principles calculations. By applying temperature difference between the source and the drain of a M-ZGNR device, spin-up and spin-down currents flowing in opposite directions can be induced. This spin Seebeck effect in M-ZGNRs can be attributed to the asymmetric electron−hole transmission spectra of spin-up and spin-down electrons. Furthermore, these spin currents can be modulated and completely polarized by tuning the back gate voltage. Finally, thermal magnetoresistance of ZGNRs between ground states and magnetized states can reach 104% without an external bias. Our results indicate the possibility of developing graphene-based spin caloritronic devices.

http://pubs.acs.org/doi/abs/10.1021/nl2000049

Low Temperature Raman Study of the Electron Coherence Length near Graphene Edges

Ryan Beams, Luiz Gustavo Canado, and Lukas Novotny

We developed a novel optical defocusing method for studying spatial coherence of photoexcited electrons and holes near edges of graphene. Our method is applied to measure the localization lD of the disorder-induced Raman D band (1350 cm−1) with a resolution of a few nanometers. Raman scattering experiments performed in a helium bath cryostat reveal that as temperature is decreased from 300 to 1.55 K, the length lD increases. We found that the localization of the D band varies as 1/T1/2, giving strong evidence that lD scales with the coherence length of photoexcited electrons near graphene edges.

http://pubs.acs.org/doi/abs/10.1021/nl104134a

Noise properties of a resonance-type spin-torque microwave detector

Oleksandr Prokopenko, Gennadiy Melkov, Elena Bankowski, Thomas Meitzler, Vasil Tiberkevich, Andrei Slavin

We analyze performance of a resonance-type spin-torque microwave detector (STMD) in the presence of noise and reveal two distinct regimes of STMD operation. In the first (high-frequency) regime the minimum detectable microwave power $P_{\rm min}$ is limited by the low-frequency Johnson-Nyquist noise and the signal-to-noise ratio (SNR) of STMD is proportional to the input microwave power $P_{\rm RF}$. In the second (low-frequency) regime $P_{\rm min}$ is limited by the magnetic noise, and the SNR is proportional to $\sqrt{P_{\rm RF}}$. The developed formalism can be used for the optimization of the practical noise-handling parameters of a STMD.

http://arxiv.org/abs/1102.5048

Thermally assisted spin transfer torque switching in synthetic free layers

Tomohiro Taniguchi, Hiroshi Imamura

We studied the magnetization reversal rates of thermally assisted spin transfer torque switching in a ferromagnetically coupled synthetic free layer theoretically. By solving the Fokker-Planck equation, we obtained the analytical expression of the switching probability for both the weak and the strong coupling limit. We found that the thermal stability is proportional to Delta_{0}(1-I/I_{c})^{2}, not Delta_{0}(1-I/I_{c}) argued by Koch et al. [Phys. Rev. Lett. 92, 088302 (2004)], where I and I_{c} are the electric current and the critical current of spin transfer torque switching at absolute zero temperature. The difference in the exponent of (1-I/I_{c}) leads to a significant underestimation of the thermal stability Delta_{0}. We also found that fast switching is achieved by choosing the appropriate direction of the applied field.

http://arxiv.org/abs/1010.5845

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

We provide a direct proof of two-electron Andreev transitions in a superconductor - normal metal tunnel junction by detecting them in a real-time electron counting experiment. Our results are consistent with ballistic Andreev transport with an order of magnitude higher rate than expected for a uniform barrier, suggesting that only part of the interface is effectively contributing to the transport. These findings are quantitatively supported by our direct current measurements in single-electron transistors with similar tunnel barriers.

http://arxiv.org/abs/1012.5750

Memory effects in complex materials and nanoscale systems

Yuriy V. Pershin, Massimiliano Di Ventra

Memory effects are ubiquitous in nature and are particularly relevant at the nanoscale where the dynamical properties of electrons and ions strongly depend on the history of the system, at least within certain time scales. We review here the memory properties of various materials and systems which appear most strikingly in their non-trivial time-dependent resistive, capacitative and inductive characteristics. We describe these characteristics within the framework of memristors, memcapacitors and meminductors, namely memory circuit elements whose properties depend on the history and state of the system. We examine basic issues related to such systems and critically report on both theoretical and experimental progress in understanding their functionalities. We also discuss possible applications of memory effects in various areas of science and technology ranging from digital to analog electronics, biologically-inspired circuits, and learning. We finally discuss future research opportunities in the field.

http://arxiv.org/abs/1011.3053v1

Feb 17. - Feb. 23. (2011)

Válogatta: Balogh Zoltán

Current quantization in an optically driven electron pump based on self-assembled quantum dots

L. Nevou, V. Liverini, P. Friedli, F. Castellano, A. Bismuto, H. Sigg, F. Gramm, E. Müller & J. Faist

The electronic structure of self-assembled semiconductor quantum dots consists of discrete atom-like states that can be populated with a well-defined number of electrons. This property can be used to fabricate a d.c. current standard that enables the unit of ampere to be independently defined. Here we report an optically pumped current source based on self-assembled InAs/GaAs quantum dots. The accuracy obtained so far is 10−1 and is limited by the uncertainty in the number of dots. At 10 K the device generates a current difference of 2.39 nA at a frequency of 1 kHz. The accuracy could be improved by site-selective growth techniques where the number of dots is fixed by pre-patterning. The results are promising for applications in electrical metrology, where a current standard is needed to close the so-called quantum metrological triangle.

http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1918.html

Giant magnetoresistance through a single molecule

Stefan Schmaus, Alexei Bagrets, Yasmine Nahas, Toyo K. Yamada, Annika Bork, Martin Bowen, Eric Beaurepaire, Ferdinand Evers, & Wulf Wulfhekel

Magnetoresistance is a change in the resistance of a material system caused by an applied magnetic field. Giant magnetoresistance occurs in structures containing ferromagnetic contacts separated by a metallic non-magnetic spacer, and is now the basis of read heads for hard drives and for new forms of random access memory. Using an insulator (for example, a molecular thin film) rather than a metal as the spacer gives rise to tunnelling magnetoresistance, which typically produces a larger change in resistance for a given magnetic field strength, but also yields higher resistances, which are a disadvantage for real device operation. Here, we demonstrate giant magnetoresistance across a single, non-magnetic hydrogen phthalocyanine molecule contacted by the ferromagnetic tip of a scanning tunnelling microscope. We measure the magnetoresistance to be 60% and the conductance to be 0.26G0, where G0 is the quantum of conductance. Theoretical analysis identifies spin-dependent hybridization of molecular and electrode orbitals as the cause of the large magnetoresistance.

http://www.nature.com/nnano/journal/vaop/ncurrent/abs/nnano.2011.11.html

Controlling single-molecule conductance through lateral coupling of π orbitals

Ismael Diez-Perez, Joshua Hihath, Thomas Hines, Zhong-Sheng Wang, Gang Zhou, Klaus Müllen & Nongjian Tao

In recent years, various single-molecule electronic components have been demonstrated1. However, it remains difficult to predict accurately the conductance of a single molecule and to control the lateral coupling between the π orbitals of the molecule and the orbitals of the electrodes attached to it. This lateral coupling is well known to cause broadening and shifting of the energy levels of the molecule; this, in turn, is expected to greatly modify the conductance of an electrode–molecule–electrode junction2, 3, 4, 5, 6. Here, we demonstrate a new method, based on lateral coupling, to mechanically and reversibly control the conductance of a single-molecule junction by mechanically modulating the angle between a single pentaphenylene molecule bridged between two metal electrodes. Changing the angle of the molecule from a highly tilted state to an orientation nearly perpendicular to the electrodes changes the conductance by an order of magnitude, which is in qualitative agreement with theoretical models of molecular π-orbital coupling to a metal electrode. The lateral coupling is also directly measured by applying a fast mechanical perturbation in the horizontal plane, thus ruling out changes in the contact geometry or molecular conformation as the source for the conductance change.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.20.html

Signatures of Disorder in the Minimum Conductivity of Graphene

Yang Sui, Tony Low, Mark Lundstrom, and Joerg Appenzeller

Graphene has been proposed as a promising material for future nanoelectronics because of its unique electronic properties. Understanding the scaling behavior of this new nanomaterial under common experimental conditions is of critical importance for developing graphene-based nanoscale devices. We present a comprehensive experimental and theoretical study on the influence of edge disorder and bulk disorder on the minimum conductivity of graphene ribbons. For the first time, we discovered a strong nonmonotonic size scaling behavior featuring a peak and saturation minimum conductivity. Through extensive numerical simulations and analysis, we are able to attribute these features to the amount of edge and bulk disorder in graphene devices. This study elucidates the quantum transport mechanisms in realistic experimental graphene systems, which can be used as a guideline for designing graphene-based nanoscale devices with improved performance.

http://pubs.acs.org/doi/abs/10.1021/nl104399z

Low-Temperature Phase Transformation from Graphite to sp3 Orthorhombic Carbon

Jian-Tao Wang, Changfeng Chen, and Yoshiyuki Kawazoe

We identify by ab initio calculations an orthorhombic carbon polymorph in Pnma symmetry that has the lowest enthalpy among proposed cold-compressed graphite phases. This new phase contains alternating zigzag and armchair buckled carbon sheets transformed via a one-layer by three-layer slip mechanism. It has a wide indirect band gap and a large bulk modulus that are comparable to those of diamond. Its simulated x-ray diffraction pattern best matches the experimental data. Pressure plays a key role in lowering the kinetic barrier during the phase conversion process. These results provide a comprehensive understanding and an excellent account for experimental findings.

http://prl.aps.org/abstract/PRL/v106/i7/e075501

Aharonov-Bohm Oscillations Changed by Indirect Interdot Tunneling via Electrodes in Parallel-Coupled Vertical Double Quantum Dots

T. Hatano1, T. Kubo1, Y. Tokura, S. Amaha1, S. Teraoka, and S. Tarucha

Aharonov-Bohm (AB) oscillations are studied for a parallel-coupled vertical double quantum dot with a common source and drain electrode. We observe AB oscillations of current via a one-electron bonding state as the ground state and an antibonding state as the excited state. As the center gate voltage becomes more negative, the oscillation period is clearly halved for both the bonding and antibonding states, and the phase changes by half a period for the antibonding state. This result can be explained by a calculation that takes account of the indirect interdot coupling via the two electrodes.

http://prl.aps.org/abstract/PRL/v106/i7/e076801

The Effect of Interlayer Adhesion on the Mechanical Behaviors of Macroscopic Graphene Oxide Papers

Yun Gao, Lu-Qi Liu, Sheng-Zhen Zu, Ke Peng, Ding Zhou, Bao-Hang Han, and Zhong Zhang

High mechanical performances of macroscopic graphene oxide (GO) papers are attracting great interest owing to their merits of lightweight and multiple functionalities. However, the loading role of individual nanosheets and its effect on the mechanical properties of the macroscopic GO papers are not yet well understood. Herein, we effectively tailored the interlayer adhesions of the GO papers by introducing small molecules, that is, glutaraldehyde (GA) and water molecules, into the gallery regions. With the help of in situ Raman spectroscopy, we compared the varied load-reinforcing roles of nanosheets, and further predicted the Young’s moduli of the GO papers. Systematic mechanical tests have proven that the enhancement of the tensile modulus and strength of the GA-treated GO paper arose from the improved load-bearing capability of the nanosheets. On the basis of Raman and macroscopic mechanical tests, the influences of interlayer adhesions on the fracture mechanisms of the strained GO papers were inferred.

http://pubs.acs.org/doi/abs/10.1021/nn103331x

Feb 9. - Feb. 16. (2011)

Válogatta: Makk Péter

Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride

Jiamin Xue, Javier Sanchez-Yamagishi,Danny Bulmash, Philippe Jacquod, Aparna Deshpande, K. Watanabe, T. Taniguchi, Pablo Jarillo-Herrero & Brian J. LeRoy

Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy–momentum dispersion relations which cross at the Dirac point1, 2. However, accessing the physics of the low-density region at the Dirac point has been difficult because of disorder that leaves the graphene with local microscopic electron and hole puddles3, 4, 5. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult6, 7. Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance8. Here we use scanning tunnelling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moiré patterns. However, contrary to predictions9, 10, this conformation does not lead to a sizeable band gap because of the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron–hole charge fluctuations are reduced by two orders of magnitude as compared with those on silicon oxide. This leads to charge fluctuations that are as small as in suspended graphene6, opening up Dirac point physics to more diverse experiments.

http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat2968.html

Gate-controlled guiding of electrons in graphene

J. R. Williams, Tony Low, M. S. Lundstrom & C. M. Marcus

Ballistic semiconductor structures have allowed the realization of optics-like phenomena in electronic systems, including the magnetic focusing1 and electrostatic lensing2 of electrons. An extension that appears unique to graphene is to use both n and p carrier types to create electronic analogues of optical devices with both positive and negative indices of refraction3. Here, we use the gate-controlled density of both p and n carrier types in graphene to demonstrate the electronic analogue of fibre-optic guiding4, 5, 6, 7, 8. Two basic effects are investigated: bipolar p–n junction guiding, based on the principle of angle-selective transmission through the interface between the graphene and the p–n junction; and unipolar fibre-optic guiding, using total internal reflection controlled by carrier density. We also demonstrate modulation of the guiding efficiency through gating, and comparison of these data with numerical simulations indicates that guiding performance is limited by the roughness of the interface. The development of p–n and fibre-optic guiding in graphene may lead to electrically reconfigurable wiring in high-mobility devices.

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2011.3.html

Programmable nanowire circuits for nanoprocessors

Hao Yan, Hwan Sung Choe, SungWoo Nam,Yongjie Hu, Shamik Das, James F. Klemic, James C. Ellenbogen & Charles M. Lieber

A nanoprocessor constructed from intrinsically nanometre-scale building blocks is an essential component for controlling memory, nanosensors and other functions proposed for nanosystems assembled from the bottom up1, 2, 3. Important steps towards this goal over the past fifteen years include the realization of simple logic gates with individually assembled semiconductor nanowires and carbon nanotubes1, 4, 5, 6, 7, 8, but with only 16 devices or fewer and a single function for each circuit. Recently, logic circuits also have been demonstrated that use two or three elements of a one-dimensional memristor array9, although such passive devices without gain are difficult to cascade. These circuits fall short of the requirements for a scalable, multifunctional nanoprocessor10, 11 owing to challenges in materials, assembly and architecture on the nanoscale. Here we describe the design, fabrication and use of programmable and scalable logic tiles for nanoprocessors that surmount these hurdles. The tiles were built from programmable, non-volatile nanowire transistor arrays. Ge/Si core/shell nanowires12 coupled to designed dielectric shells yielded single-nanowire, non-volatile field-effect transistors (FETs) with uniform, programmable threshold voltages and the capability to drive cascaded elements. We developed an architecture to integrate the programmable nanowire FETs and define a logic tile consisting of two interconnected arrays with 496 functional configurable FET nodes in an area of ~960 μm2. The logic tile was programmed and operated first as a full adder with a maximal voltage gain of ten and input–output voltage matching. Then we showed that the same logic tile can be reprogrammed and used to demonstrate full-subtractor, multiplexer, demultiplexer and clocked D-latch functions. These results represent a significant advance in the complexity and functionality of nanoelectronic circuits built from the bottom up with a tiled architecture that could be cascaded to realize fully integrated nanoprocessors with computing, memory and addressing capabilities.

http://www.nature.com/nature/journal/v470/n7333/full/nature09749.html

Realizing Lateral Wrap-Gated Nanowire FETs: Controlling Gate Length with Chemistry Rather than Lithography

Kristian Storm†, Gustav Nylund†, Lars Samuelson†, and Adam P. Micolich

An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A nanowire with a coaxial metal gate provides optimal gate-channel coupling but has only been realized for vertically oriented nanowire transistors. We report a method for producing laterally oriented wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. It allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the subthreshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 and 4 K, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing.

http://pubs.acs.org/doi/abs/10.1021/nl104403g

Gate-Induced Fermi Level Tuning in InP Nanowires at Efficiency Close to the Thermal Limit

Kristian Storm, Gustav Nylund, Magnus Borgstrm, Jesper Wallentin, Carina Fasth, Claes Thelander, and Lars Samuelson

As downscaling of semiconductor devices continues, one or a few randomly placed dopants may dominate the characteristics. Furthermore, due to the large surface-to-volume ratio of one-dimensional devices, the position of the Fermi level is often determined primarily by surface pinning, regardless of doping level. In this work, we investigate the possibility of tuning the Fermi level dynamically with wrap-around gates, instead of statically setting it using the impurity concentration. This is done using Ω-gated metal−oxide−semiconductor field-effect transistors with HfO2-capped InP nanowires as channel material. It is found that induced n-type devices exhibit an optimal inverse subthreshold slope of 68 mV/decade. By adjusting the growth and process parameters, it is possible to produce ambipolar devices, in which the Fermi level can be tuned across the entire band gap, making it possible to induce both n-type and p-type conduction.

http://pubs.acs.org/doi/abs/10.1021/nl104032s

Probing the Gate−Voltage-Dependent Surface Potential of Individual InAs Nanowires Using Random Telegraph Signal

Joe Salfi†§*, Nicola Paradiso‡, Stefano Roddaro‡, Stefan Heun‡, Selvakumar V. Nair†, Igor G. Savelyev†, Marina Blumin†, Fabio Beltram‡, and Harry E. Ruda

We report a novel method for probing the gate-voltage dependence of the surface potential of individual semiconductor nanowires. The statistics of electronic occupation of a single defect on the surface of the nanowire, determined from a random telegraph signal, is used as a measure for the local potential. The method is demonstrated for the case of one or two switching defects in indium arsenide (InAs) nanowire field effect transistors at temperatures T = 25−77 K. Comparison with a self-consistent model shows that surface potential variation is retarded in the conducting regime due to screening by surface states with density Dss = 1 − 2 × 1012 cm−2 eV−1. Temperature-dependent dynamics of electron capture and emission producing the random telegraph signals are also analyzed, and multiphonon emission is identified as the process responsible for capture and emission of electrons from the surface traps. Two defects studied in detail had capture activation energies of EB ≈ 50 meV and EB ≈ 110 meV and cross sections of σ∞ ≈ 3 × 10−19 cm2 and σ∞ ≈ 2 × 10−17 cm2, respectively. A lattice relaxation energy of Sω = 187 ± 15 meV was found for the first defect.

http://pubs.acs.org/doi/abs/10.1021/nn1033967

Raman 2D-Band Splitting in Graphene: Theory and Experiment

Otakar Frank†‡, Marcel Mohr§, Janina Maultzsch§, Christian Thomsen§, Ibtsam Riaz, Rashid Jalil, Kostya S. Novoselov, Georgia Tsoukleri†, John Parthenios†, Konstantinos Papagelis†¶*, Ladislav Kavan‡, and Costas Galiotis

We present a systematic experimental and theoretical study of the two-phonon (2D) Raman scattering in graphene under uniaxial tension. The external perturbation unveils that the 2D mode excited with 785 nm has a complex line-shape mainly due to the contribution of two distinct double resonance scattering processes (inner and outer) in the Raman signal. The splitting depends on the direction of the applied strain and the polarization of the incident light. The results give new insight into the nature of the 2D band and have significant implications for the use of graphene as reinforcement in composites since the 2D mode is crucial to assess how effectively graphene uptakes an applied stress or strain.

http://pubs.acs.org/doi/abs/10.1021/nn103493g

Inelastic Electron Tunneling Spectroscopy of Gold−Benzenedithiol−Gold Junctions: Accurate Determination of Molecular Conformation

Li-Li Lin†‡, Chuan-Kui Wang†, and Yi Luo

The gold−benzenedithiol−gold junction is the classic prototype of molecular electronics. However, even with the similar experimental setup, it has been difficult to reproduce the measured results because of the lack of basic information about the molecular confirmation inside the junction. We have performed systematic first principles study on the inelastic electron tunneling spectroscopy of this classic junction. By comparing the calculated spectra with four different experimental results, the most possible conformations of the molecule under different experimental conditions have been successfully determined. The relationship between the contact configuration and the resulted spectra is revealed. It demonstrates again that one should always combine the theoretical and experimental inelastic electron tunneling spectra to determine the molecular conformation in a junction. Our simulations have also suggested that in terms of the reproducibility and stability, the electromigrated nanogap technique is much better than the mechanically controllable break junction technique.

http://pubs.acs.org/doi/abs/10.1021/nn103522k

Collapse of Landau Levels in Gated Graphene Structures

Nan Gu1, Mark Rudner2, Andrea Young3, Philip Kim3, and Leonid Levitov

We describe a new regime of magnetotransport in two-dimensional electron systems in the presence of a narrow potential barrier. In such systems, the Landau level states, which are confined to the barrier region in strong magnetic fields, undergo a deconfinement transition as the field is lowered. Transport measurements on a top-gated graphene device are presented. Shubnikov–de Haas (SdH) oscillations, observed in the unipolar regime, are found to abruptly disappear when the strength of the magnetic field is reduced below a certain critical value. This behavior is explained by a semiclassical analysis of the transformation of closed cyclotron orbits into open, deconfined trajectories.

http://prl.aps.org/abstract/PRL/v106/i6/e066601

Atomic force microscope nanolithography of graphene: cuts, pseudo-cuts and tip current measurements

R.K. Puddy, P.H. Scard, D. Tyndall, M.R. Connolly, C.G. Smith, G.A.C. Jones, A. Lombardo, A.C. Ferrari, M.R. Buitelaar

We investigate atomic force microscope nanolithography of single and bilayer graphene. In situ tip current measurements show that cutting of graphene is not current driven. Using a combination of transport measurements and scanning electron microscopy we show that, while indentations accompanied by tip current appear in the graphene lattice for a range of tip voltages, real cuts are characterized by a strong reduction of the tip current above a threshold voltage. The reliability and flexibility of the technique is demonstrated by the fabrication, measurement, modification and re-measurement of graphene nanodevices with resolution down to 15 nm.

http://arxiv.org/abs/1102.2781

Magnetoresistance through a single molecule

Stefan Schmaus, Alexei Bagrets, Yasmine Nahas, Toyo K. Yamada, Annika Bork, Martin Bowen, Eric Beaurepaire, Ferdinand Evers, Wulf Wulfhekel

The use of single molecules to design electronic devices is an extremely challenging and fundamentally different approach to further downsizing electronic circuits. Two-terminal molecular devices such as diodes were first predicted [1] and, more recently, measured experimentally [2]. The addition of a gate then enabled the study of molecular transistors [3-5]. In general terms, in order to increase data processing capabilities, one may not only consider the electron's charge but also its spin [6,7]. This concept has been pioneered in giant magnetoresistance (GMR) junctions that consist of thin metallic films [8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains, however, a challenging endeavor. As an important first step in this field, we have performed an experimental and theoretical study on spin transport across a molecular GMR junction consisting of two ferromagnetic electrodes bridged by a single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can enhance the magnetoresistance by one order of magnitude to 52%.

http://arxiv.org/abs/1102.2630

Strong-coupling topological Josephson effect in quantum wires

Flavio S. Nogueira, Ilya Eremin

the Josephson effect for a setup with two lattice quantum wires featuring Majorana zero energy boundary modes at the tunnel junction. In the weak-coupling, the exact solution reproduces the perturbative result for the energy containing a contribution $\sim \pm\cos(\phi/2)$ relative to the tunneling of paired Majorana fermions. As the tunnel amplitude $g$ grows relative to the hopping amplitude $w$, the gap between the energy levels gradually diminishes until it closes completely at the critical value $g_c=\sqrt{2}w$. At this point the Josephson energies are $E_{m\sigma}=2\sigma\sqrt{2}w\cos(\phi/6-\pi m/3)$, where $m=-1,0,1$ and $\sigma=\pm 1$, which is very different from the result obtained at weak-coupling. It represents the merging of three Bogolyubov states, leading to additional degeneracies and a topologically nontrivial ground state.

http://arxiv.org/abs/1102.3000

Jan. 26. - Feb. 8. (2011)

Válogatta: Makk Péter és Geresdi Attila

Transport through Andreev bound states in a graphene quantum dot

Travis Dirks, Taylor L. Hughes, Siddhartha Lal, Bruno Uchoa, Yung-Fu Chen, Cesar Chialvo, Paul M. Goldbart and Nadya Mason

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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@@ Line 54: / Line 54: @@
 http://pubs.acs.org/doi/abs/10.1021/nn200114p
 ----
+'''Imaging Universal Conductance Fluctuations in Graphene'''
+Mario F. Borunda, Jesse Berezovsky, Robert M. Westervelt, and Eric J. Heller
+We study conductance fluctuations (CF) and the sensitivity of the conductance to the motion of a single scatterer in two-dimensional massless Dirac systems. Our extensive numerical study finds limits to the predicted universal value of CF. We find that CF are suppressed for ballistic systems near the Dirac point and approach the universal value at sufficiently strong disorder. The conductance of massless Dirac fermions is sensitive to the motion of a single scatterer. CF of order e2/h result from the motion of a single impurity by a distance comparable to the Fermi wavelength. This result applies to graphene systems with a broad range of impurity strength and concentration while the dependence on the Fermi wavelength can be explored via gate voltages. Our prediction can be tested by comparing graphene samples with varying amounts of disorder and can be used to understand interference effects in mesoscopic graphene devices.
+http://pubs.acs.org/doi/abs/10.1021/nn103450d
+----
+'''Giant Nonlocality Near the Dirac Point in Graphene'''
+D. A. Abanin, S. V. Morozov, L. A. Ponomarenko, R. V. Gorbachev, A. S. Mayorov, M. I. Katsnelson, K. Watanabe, T. Taniguchi, K. S. Novoselov, L. S. Levitov, A. K. Geim
+Transport measurements have been a powerful tool for discovering electronic phenomena in graphene. We report nonlocal measurements performed in the Hall bar geometry with voltage probes far away from the classical path of charge flow. We observed a large nonlocal response near the Dirac point in fields as low as 0.1 tesla, which persisted up to room temperature. The nonlocality is consistent with the long-range flavor currents induced by the lifting of spin/valley degeneracy. The effect is expected to contribute strongly to all magnetotransport phenomena near the neutrality point.
+http://www.sciencemag.org/content/332/6027/328.abstract
+-----
+'''Bottom-gated epitaxial graphene'''
+Daniel Waldmann, Johannes Jobst, Florian Speck, Thomas Seyller, Michael Krieger, Heiko B. Weber
+High-quality epitaxial graphene on silicon carbide (SiC) is today available in wafer size1, 2. Similar to exfoliated graphene3, 4, its charge carriers are governed by the Dirac–Weyl Hamiltonian5, 6, 7 and it shows excellent mobilities7, 8. For many experiments with graphene, in particular for surface science, a bottom gate is desirable9, 10, 11, 12, 13. Commonly, exfoliated graphene flakes are placed on an oxidized silicon wafer that readily provides a bottom gate3, 4, 11. However, this cannot be applied to epitaxial graphene as the SiC provides the source material out of which graphene grows. Here, we present a reliable scheme for the fabrication of bottom-gated epitaxial graphene devices, which is based on nitrogen (N) implantation into a SiC wafer and subsequent graphene growth. We demonstrate working devices in a broad temperature range from 6 to 300 K. Two gating regimes can be addressed, which opens a wide engineering space for tailored devices by controlling the doping of the gate structure.
+http://www.nature.com/nmat/journal/v10/n5/full/nmat2988.html
+----
+'''Drude conductivity of Dirac fermions in graphene'''
+Jason Horng, Chi-Fan Chen, Baisong Geng, Caglar Girit, Yuanbo Zhang, Zhao Hao, Hans A. Bechtel, Michael Martin, Alex Zettl, Michael F. Crommie, Y. Ron Shen, and Feng Wang
+Electrons moving in graphene behave as massless Dirac fermions, and they exhibit fascinating low-frequency electrical transport phenomena. Their dynamic response, however, is little known at frequencies above one terahertz (THz). Such knowledge is important not only for a deeper understanding of the Dirac electron quantum transport, but also for graphene applications in ultrahigh-speed THz electronics and infrared (IR) optoelectronics. In this paper, we report measurements of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations. The conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies, but its absolute strength is lower than theoretical predictions. This anomalous reduction of free-electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum rule.
+http://prb.aps.org/abstract/PRB/v83/i16/e165113
+----
+'''Acoustic phonons in graphene nanoribbons'''
+Matthias Droth, Guido Burkard
+Phonons are responsible for limiting both the carrier mobility and the spin relaxation time. In view of a possible transistor function as well as spintronics applications in graphene nanoribbons, we present a theoretical study of acoustic phonons in these nanostructures. Using a two-dimensional continuum model which takes into accout the monatomic thickness of graphene, we derive hermitian wave equations as well as phonon creation and annihilation operators, thus allowing for a quantum mechanical treatment of the system. Two types of boundary configurations, which we believe can be realized in experiment, are discussed: (i) fixed and (ii) free boundaries. The former leads to a gapped phonon dispersion relation while the latter exhibits an ungapped dispersion relation and a finite sound velocity of out-of-plane modes at the center of the Brillouin zone. In the limit of negligible boundary effects, bulk-like behaviour is restored.
+http://arxiv.org/abs/1104.3520
+----
+'''Localized Andreev edge states in HgTe quantum wells.'''
+I.M. Khaymovich, N.M. Chtchelkatchev, V.M. Vinokur
+We investigate the interplay of superconductivity and the topological order in the topological insulator (TI) formed in HgTe-CdTe quantum wells coupled to the s-wave isotropic superconductor (SC) placed on top of CdTe layer. Proximity effect induces superconducting correlations in TI effecting classification of the TI electronic states. Depending on the strength of the coupling and the initial chemical potentials in SC and TI, the edge states either (i) become gapped, or (ii) hybridize into localized Andreev states, or else (iii) merge with the bulk states turning the TI into an anisotropic narrow-gap semiconductor.
+arXiv:1104.3499v1
+----
+'''Quantum Hall Effect in Thin Films of Three-Dimensional Topological Insulators.'''
+Huichao Li, L. Sheng, D. Y. Xing
+We show that a thin film of a three-dimensional topological insulator (3DTI) with an exchange field is a realization of the famous Haldane model for quantum Hall effect (QHE) without Landau levels. The exchange field plays the role of staggered fluxes on the honeycomb lattice, and the hybridization gap of the surface states is equivalent to alternating on-site energies on the AB sublattices. A peculiar phase diagram for the QHE is predicted in 3DTI thin films under an applied magnetic field, which is quite different from that either in traditional QHE systems or in graphene.
+arXiv:1104.3407v1
+----
+'''Helical States of Topological Insulator Bi2Se3'''
+Yonghong Zhao, Yibin Hu, Lei Liu, Yu Zhu, and Hong Guo
+We report density functional theory analysis of the electronic and quantum transport properties of Bi2Se3 topological insulator, focusing on the helical surface states at the Fermi level EF. The calculated Dirac point and the tilt angle of the electron spin in the helical states are compared quantitatively with the experimental data. The calculated conductance near EF shows a V-shaped spectrum, consistent with STM measurements. The spins in the helical states at EF not only tilts out of the two-dimensional plane, they also oscillate with a 3-fold symmetry going around the two-dimensional Brillouin zone. The helical states penetrate into the material bulk, where the first quintuple layer contributes 70% of the helical wave functions.
+http://pubs.acs.org/doi/abs/10.1021/nl200584f
+----
+'''Observation of topological order in a Superconducting doped topological insulator (based on the Bi2Se3 class).'''
+L. Andrew Wray, Suyang Xu, Yuqi Xia, Dong Qian, Alexei V. Fedorov, Hsin Lin, Arun Bansil, Yew San Hor, Robert J. Cava, M. Zahid Hasan
+Topological insulators embody a new state of matter characterized entirely by the topological invariants of the bulk electronic structure rather than any form of spontaneously broken symmetry. Unlike the 2D quantum Hall or quantum spin-Hall-like systems, the three dimensional (3D) topological insulators can host magnetism and superconductivity which has generated widespread research activity in condensed-matter and materials-physics communities. Thus there is an explosion of interest in understanding the rich interplay between topological and the broken-symmetry states (such as superconductivity), greatly spurred by proposals that superconductivity introduced into certain band structures will host exotic quasiparticles which are of interest in quantum information science. The observations of superconductivity in doped Bi_2Se_3 (Cu$_x$Bi$_2$Se$_3$) and doped Bi_2Te_3 (Pd$_x$-Bi$_2$Te$_3$ T$_c$ $\sim$ 5K) have raised many intriguing questions about the spin-orbit physics of these ternary complexes while any rigorous theory of superconductivity remains elusive. Here we present key measurements of electron dynamics in systematically tunable normal state of Cu$_x$Bi$_2$Se$_3$ (x=0 to 12%) gaining insights into its spin-orbit behavior and the topological nature of the surface where superconductivity takes place at low temperatures. Our data reveal that superconductivity occurs (in sample compositions) with electrons in a bulk relativistic kinematic regime and we identify that an unconventional doping mechanism causes the topological surface character of the undoped compound to be preserved at the Fermi level of the superconducting compound, where Cooper pairing occurs at low temperatures. These experimental observations provide important clues for developing a theory of topological-superconductivity in 3D topological insulators.
+arXiv:1104.3881v1
+----
+'''Surface-Quantized Anomalous Hall Current and the Magnetoelectric Effect in Magnetically Disordered Topological Insulators'''
+Kentaro Nomura and Naoto Nagaosa
+We study theoretically the role of quenched magnetic disorder at the surface of topological insulators by numerical simulation and scaling analysis based on the massive Dirac fermion model. This addresses the problem of Anderson localization on chiral anomaly. It is found that all the surface states are localized, while the transverse conductivity is quantized to be ±e2/2h as long as the Fermi energy is within the bulk gap. This greatly facilitates the realization of the topological magnetoelectric effect proposed by Qi et al. [Phys. Rev. B 78, 195424 (2008)] with the surface magnetization direction being controlled by the simultaneous application of magnetic and electric fields.
+http://prl.aps.org/abstract/PRL/v106/i16/e166802
+----
+'''Competing exotic topological insulator phases in transition-metal oxides on the pyrochlore lattice with distortion'''
+Mehdi Kargarian, Jun Wen, and Gregory A. Fiete
+In this work we investigate the phase diagram of heavy (4d and 5d) transition-metal oxides on the pyrochlore lattice, such as those of the form A2M2O7, where A is a rare-earth element and M is a transition-metal element. We focus on the competition between Coulomb interaction, spin-orbit coupling, and lattice distortion when these energy scales are comparable. Strong spin-orbit coupling entangles the spin and the t2g d orbitals giving rise to doublet j=1/2 and quadruplet j=3/2 states. In contrast to previous works which focused on the doublet manifold, we also discuss the quadruplet manifold which is relevant for several pyrochlore oxides. The Coulomb interaction is taken into account by use of the slave-rotor mean-field theory and different classes of lattice distortions which further split the levels of the quadruplet j=3/2 manifold are studied. Various topological phases are predicted, including exotic strong and weak topological Mott insulating phases. We discuss the general structure of the phase diagram for several values of d-shell filling and various symmetry classes of lattice distortions. Our results are relevant to the search for exotic topological insulators and quantum spin liquids in strongly correlated materials with strong spin-orbit coupling.
+http://prb.aps.org/abstract/PRB/v83/i16/e165112
+-----
+'''Quantum simulation of antiferromagnetic spin chains in an optical lattice'''
+Jonathan Simon, Waseem S. Bakr, Ruichao Ma, M. Eric Tai, Philipp M. Preiss & Markus Greiner
+Understanding exotic forms of magnetism in quantum mechanical systems is a central goal of modern condensed matter physics, with implications for systems ranging from high-temperature superconductors to spintronic devices. Simulating magnetic materials in the vicinity of a quantum phase transition is computationally intractable on classical computers, owing to the extreme complexity arising from quantum entanglement between the constituent magnetic spins. Here we use a degenerate Bose gas of rubidium atoms confined in an optical lattice to simulate a chain of interacting quantum Ising spins as they undergo a phase transition. Strong spin interactions are achieved through a site-occupation to pseudo-spin mapping. As we vary a magnetic field, quantum fluctuations drive a phase transition from a paramagnetic phase into an antiferromagnetic phase. In the paramagnetic phase, the interaction between the spins is overwhelmed by the applied field, which aligns the spins. In the antiferromagnetic phase, the interaction dominates and produces staggered magnetic ordering. Magnetic domain formation is observed through bothin situ site-resolved imaging and noise correlation measurements. By demonstrating a route to quantum magnetism in an optical lattice, this work should facilitate further investigations of magnetic models using ultracold atoms, thereby improving our understanding of real magnetic materials.
+http://www.nature.com/nature/journal/v472/n7343/full/nature09994.html
+----
+'''Teleportation of Nonclassical Wave Packets of Light'''
+Noriyuki Lee, Hugo Benichi, Yuishi Takeno, Shuntaro Takeda, James Webb, Elanor Huntington, Akira Furusawa
+We report on the experimental quantum teleportation of strongly nonclassical wave packets of light. To perform this full quantum operation while preserving and retrieving the fragile nonclassicality of the input state, we have developed a broadband, zero-dispersion teleportation apparatus that works in conjunction with time-resolved state preparation equipment. Our approach brings within experimental reach a whole new set of hybrid protocols involving discrete- and continuous-variable techniques in quantum information processing for optical sciences.
+http://www.sciencemag.org/content/332/6027/330.abstract
+----
+'''Infrared-spectroscopic nanoimaging with a thermal source'''
+F. Huth, M. Schnell, J. Wittborn, N. Ocelic & R. HillenbrandFourier-transform infrared (FTIR) spectroscopy is a widely used analytical tool for chemical identification of inorganic, organic and biomedical materials, as well as for exploring conduction phenomena. Because of the diffraction limit, however, conventional FTIR cannot be applied for nanoscale imaging. Here we demonstrate a novel FTIR system that allows for infrared-spectroscopic nanoimaging of dielectric properties (nano-FTIR). Based on superfocusing of thermal radiation with an infrared antenna, detection of the scattered light, and strong signal enhancement employing an asymmetric FTIR spectrometer, we improve the spatial resolution of conventional infrared spectroscopy by more than two orders of magnitude. By mapping a semiconductor device, we demonstrate spectroscopic identification of silicon oxides and quantification of the free-carrier concentration in doped Si regions with a spatial resolution better than 100 nm. We envisage nano-FTIR becoming a powerful tool for chemical identification of nanomaterials, as well as for quantitative and contact-free measurement of the local free-carrier concentration and mobility in doped nanostructures.
+http://www.nature.com/nmat/journal/v10/n5/abs/nmat3006.html
+----
+'''Environmental Control of Single-Molecule Junction Transport'''
+V. Fatemi, M. Kamenetska, J. B. Neaton, and L. Venkataraman
+The conductance of individual 1,4-benzenediamine (BDA)–Au molecular junctions is measured in different solvent environments using a scanning tunneling microscope based point-contact technique. Solvents are found to increase the conductance of these molecular junctions by as much as 50%. Using first principles calculations, we explain this increase by showing that a shift in the Au contact work function is induced by solvents binding to undercoordinated Au sites around the junction. Increasing the Au contact work function reduces the separation between the Au Fermi energy and the highest occupied molecular orbital of BDA in the junction, increasing the measured conductance. We demonstrate that the solvent-induced shift in conductance depends on the affinity of the solvent to Au binding sites and also on the induced dipole (relative to BDA) upon adsorption. Via this mechanism, molecular junction level alignment and transport properties can be statistically altered by solvent molecule binding to the contact surface.
+http://pubs.acs.org/doi/abs/10.1021/nl200324e
 == Ápr. 1. - Ápr. 7. (2011) ==