Recent interesting articles

Realizing qubits that simultaneously provide long coherence times and fast control is a key challenge for quantum information processing. Spins in III/V semiconductor quantum dots can be electrically manipulated, but lose coherence due to interactions with nuclear spins. Silicon provides improved coherence because of its low and controllable concentration of nuclear spins. The Ge-Si core-shell heterostructured nanowire is an example of a predominantly zero-nuclear-spin system that is particularly tunable and scalable. Holes in Ge-Si nanowires exhibit large spin-orbit coupling, a useful resource for fast, all-electrical control of single spins. Here, we measure spin coherence times of gate-confined hole spins in a Ge-Si nanowire double quantum dot using high bandwidth electrical control and read out of the spin state. We find inhomogeneous dephasing times T∗2 up to 0.18μs, twenty times longer than in III/V semiconductors. This timescale is consistent with dephasing due to sparse 73Ge nuclear spins. The observed exponential coherence decay suggests a dephasing source with high-frequency spectral content. These results pave the way towards improved spin-orbit qubits and strong spin-cavity coupling in circuit quantum electrodynamics.

http://arxiv.org/abs/1403.2093

Superconducting proximity effect and zero-bias anomaly in transport through quantum dots weakly attached to ferromagnetic leads

'I. Weymann, P. Trocha'

The Andreev transport through a quantum dot coupled to two external ferromagnetic leads and one superconducting lead is studied theoretically by means of the real-time diagrammatic technique in the sequential and cotunneling regimes. We show that the tunnel magnetoresistance (TMR) of the Andreev current displays a nontrivial dependence on the bias voltage and the level detuning, and can be described by analytical formulas in the zero temperature limit. The cotunneling processes lead to a strong modification of the TMR, which is most visible in the Coulomb blockade regime. We find a zero-bias anomaly of the Andreev differential conductance in the parallel configuration, which is associated with a nonequilibrium spin accumulation in the dot triggered by Andreev processes.

http://arxiv.org/abs/1403.2956

Electrical spin protection and manipulation via gate-locked spin-orbit fields

'F. Dettwiler, J. Fu, S. Mack, P. J. Weigele, J. C. Egues, D. D. Awschalom, D. M. Zumbühl'

The spin-orbit (SO) interaction couples electron spin and momentum via a relativistic, effective magnetic field. While conveniently facilitating coherent spin manipulation in semiconductors, the SO interaction also inherently causes spin relaxation. A unique situation arises when the Rashba and Dresselhaus SO fields are matched, strongly protecting spins from relaxation, as recently demonstrated. Quantum computation and spintronics devices such as the paradigmatic spin transistor could vastly benefit if such spin protection could be expanded from a single point into a broad range accessible with in-situ gate-control, making possible tunable SO rotations under protection from relaxation. Here, we demonstrate broad, independent control of all relevant SO fields in GaAs quantum wells, allowing us to tune the Rashba and Dresselhaus SO fields while keeping both locked to each other using gate voltages. Thus, we can electrically control and simultaneously protect the spin. Our experiments employ quantum interference corrections to electrical conductivity as a sensitive probe of SO coupling. Finally, we combine transport data with numerical SO simulations to precisely quantify all SO terms.

http://arxiv.org/abs/1403.3518

Fibonacci Anyons From Abelian Bilayer Quantum Hall States

'Abolhassan Vaezi, Maissam Barkeshli'

The possibility of realizing non-Abelian statistics and utilizing it for topological quantum computation (TQC) has generated widespread interest. However, the non-Abelian statistics that can be realized in most accessible proposals is not powerful enough for universal TQC. In this paper, we consider a simple bilayer fractional quantum Hall (FQH) system with the 1/3 Laughlin state in each layer, in the presence of interlayer tunneling. We show that interlayer tunneling can drive a continuous phase transition to an exotic non-Abelian state that contains the famous `Fibonacci' anyon, whose non-Abelian statistics is powerful enough for universal TQC. Our analysis rests on startling agreements from a variety of distinct methods, including thin torus limits, effective field theories, and coupled wire constructions. The charge gap remains open at the transitions while the neutral gap closes. This raises the question of whether these exotic phases may have already been realized at ν=2/3 in bilayers, as past experiments may not have definitively ruled them out.

http://arxiv.org/abs/1403.3383

High Resolution Coherent Population Trapping on a Single Hole Spin in a Semiconductor Quantum Dot

'Julien Houel, Jonathan H. Prechtel, Andreas V. Kuhlmann, Daniel Brunner, Christopher E. Kuklewicz, Brian D. Gerardot, Nick G. Stoltz, Pierre M. Petroff, and Richard J. Warburton'

We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physicslike dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing. We identify the cause of this process as charge noise by, first, demonstrating that the hole spin g factor in this configuration (in-plane magnetic field) is strongly dependent on the vertical electric field, and second, by characterizing the charge noise through its effects on the optical transition frequency. An important conclusion is that charge noise is an important hole spin dephasing process.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.107401

'

Febr. 22. - Márc. 3. (2014)

Assorted by: Magyarkuti András

Sample Corrugation Affects the Apparent Bond Lengths in Atomic Force Microscopy

'Mark P. Boneschanscher, Sampsa K. Hämäläinen, Peter Liljeroth, and Ingmar Swart'

Frequency modulation atomic force microscopy (AFM) allows the chemical structure of planar molecules to be determined with atomic resolution. Typically, these measurements are carried out in constant-height mode using carbon monoxide (CO) terminated tips. Such tips exhibit considerable flexibility, i.e., the CO molecule can bend laterally due to the tip–sample interaction. Using epitaxial graphene as a model system, we demonstrate experimentally that the apparent atomic positions measured by AFM depend on the sample corrugation. Using molecular mechanics simulations, we explain these observations by the interplay of the CO bending and the nonlinear background signal arising from the neighboring atoms. These effects depend nontrivially on the tip–sample distance and limit the achievable accuracy on the bond length determination based on AFM experiments.

ACS Nano, Article ASAP DOI: 10.1021/nn500317r

http://pubs.acs.org/doi/abs/10.1021/nn500317r

Comprehensive Physical Model of Dynamic Resistive Switching in an Oxide Memristor

'Sungho Kim, ShinHyun Choi, and Wei Lu'

Memristors have been proposed for a number of applications from nonvolatile memory to neuromorphic systems. Unlike conventional devices based solely on electron transport, memristors operate on the principle of resistive switching (RS) based on redistribution of ions. To date, a number of experimental and modeling studies have been reported to probe the RS mechanism; however, a complete physical picture that can quantitatively describe the dynamic RS behavior is still missing. Here, we present a quantitative and accurate dynamic switching model that not only fully accounts for the rich RS behaviors in memristors in a unified framework but also provides critical insight for continued device design, optimization, and applications. The proposed model reveals the roles of electric field, temperature, oxygen vacancy concentration gradient, and different material and device parameters on RS and allows accurate predictions of diverse set/reset, analog switching, and complementary RS behaviors using only material-dependent device parameters.

ACS Nano, Article ASAP DOI: 10.1021/nn405827t

http://pubs.acs.org/doi/abs/10.1021/nn405827t

Probing the Spin States of a Single Acceptor Atom

'Joost van der Heijden, Joe Salfi, Jan A. Mol, Jan Verduijn, Giuseppe C. Tettamanzi, Alex R. Hamilton, Nadine Collaert, and Sven Rogge'

We demonstrate a single-hole transistor using an individual acceptor dopant embedded in a silicon channel. Magneto-transport spectroscopy reveals that the ground state splits as a function of magnetic field into four states, which is unique for a single hole bound to an acceptor in a bulk semiconductor. The two lowest spin states are heavy (|mj| = 3/2) and light (|mj| = 1/2) hole-like, a two-level system that can be electrically driven and is characterized by a magnetic field dependent and long relaxation time, which are properties of interest for qubits. Although the bulklike spin splitting of a boron atom is preserved in our nanotransistor, the measured Landé g-factors, |ghh| = 0.81 ± 0.06 and |glh| = 0.85 ± 0.21 for heavy and light holes respectively, are lower than the bulk value.

Nano Lett., Article ASAP DOI: 10.1021/nl4047015

http://pubs.acs.org/doi/abs/10.1021/nl4047015

Valley Splitting in a Silicon Quantum Device Platform

'Jill A. Miwa †, Oliver Warschkow ‡, Damien J. Carter §, Nigel A. Marks §, Federico Mazzola , Michelle Y. Simmons , and Justin W. Wells'

By suppressing an undesirable surface Umklapp process, it is possible to resolve the two most occupied states (1Γ and 2Γ) in a buried two-dimensional electron gas (2DEG) in silicon. The 2DEG exists because of an atomically sharp profile of phosphorus dopants which have been formed beneath the Si(001) surface (a δ-layer). The energy separation, or valley splitting, of the two most occupied bands has critical implications for the properties of δ-layer derived devices, yet until now, has not been directly measurable. Density functional theory (DFT) allows the 2DEG band structure to be calculated, but without experimental verification the size of the valley splitting has been unclear. Using a combination of direct spectroscopic measurements and DFT we show that the measured band structure is in good qualitative agreement with calculations and reveal a valley splitting of 132 ± 5 meV. We also report the effective mass and occupation of the 2DEG states and compare the dispersions and Fermi surface with DFT.

Nano Lett., Article ASAP DOI: 10.1021/nl404738j

http://pubs.acs.org/doi/abs/10.1021/nl404738j

Controlling Graphene Ultrafast Hot Carrier Response from Metal-like to Semiconductor-like by Electrostatic Gating

'S.-F. Shi, T.-T. Tang, B. Zeng, L. Ju, Q. Zhou, A. Zettl, and F. Wang'

We investigate the ultrafast terahertz response of electrostatically gated graphene upon optical excitation. We observe that the photoinduced terahertz absorption increases in charge neutral graphene but decreases in highly doped graphene. We show that this transition from semiconductor-like to metal-like response is unique for zero bandgap materials such as graphene. In charge neutral graphene photoexcited hot carriers effectively increase electron and hole densities and increase the conductivity. In highly doped graphene, however, photoexcitation does not change net conducting carrier concentration. Instead, it mainly increases electron scattering rate and reduce the conductivity.

Nano Lett., Article ASAP DOI: 10.1021/nl404826r

http://pubs.acs.org/doi/abs/10.1021/nl404826r

Graphene Plasmon Enhanced Vibrational Sensing of Surface-Adsorbed Layers

'Yilei Li, Hugen Yan, Damon B. Farmer, Xiang Meng §, Wenjuan Zhu, Richard M. Osgood, Tony F. Heinz, and Phaedon Avouris'

We characterize the influence of graphene nanoribbon plasmon excitation on the vibrational spectra of surface-absorbed polymers. As the detuning between the graphene plasmon frequency and a vibrational frequency of the polymer decreases, the vibrational peak intensity first increases and is then transformed into a region of narrow optical transparency as the frequencies overlap. Examples of this are provided by the carbonyl vibration in thin films of poly(methyl methacrylate) and polyvinylpyrrolidone. The signal depth of the plasmon-induced transparency is found to be 5 times larger than that of light attenuated by the carbonyl vibration alone. The plasmon-vibrational mode coupling and the resulting fields are analyzed using both a phenomenological model of electromagnetically coupled oscillators and finite-difference time-domain simulations. It is shown that this coupling and the resulting absorption enhancement can be understood in terms of near-field electromagnetic interactions.

Nano Lett., Article ASAP DOI: 10.1021/nl404824w

http://pubs.acs.org/doi/abs/10.1021/nl404824w

An antidamping spin–orbit torque originating from the Berry curvature

'H. Kurebayashi, Jairo Sinova, D. Fang, A. C. Irvine, T. D. Skinner, J. Wunderlich, V. Novák, R. P. Campion, B. L. Gallagher, E. K. Vehstedt, L. P. Zârbo, K. Výborný, A. J. Ferguson & T. Jungwirth'

Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin–orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin–orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic–paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin–orbit torque as well as its microscopic modelling. We expect the Berry curvature spin–orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect.

Nature Nanotechnology 9, 211–217 (2014) doi:10.1038/nnano.2014.15

http://www.nature.com/nnano/journal/v9/n3/full/nnano.2014.15.html

Evidence for quantum annealing with more than one hundred qubits

'Sergio Boixo, Troels F. Rønnow, Sergei V. Isakov, Zhihui Wang, David Wecker, Daniel A. Lidar, John M. Martinis & Matthias Troyer'

Quantum technology is maturing to the point where quantum devices, such as quantum communication systems, quantum random number generators and quantum simulators may be built with capabilities exceeding classical computers. A quantum annealer, in particular, solves optimization problems by evolving a known initial configuration at non-zero temperature towards the ground state of a Hamiltonian encoding a given problem. Here, we present results from tests on a 108 qubit D-Wave One device based on superconducting flux qubits. By studying correlations we find that the device performance is inconsistent with classical annealing or that it is governed by classical spin dynamics. In contrast, we find that the device correlates well with simulated quantum annealing. We find further evidence for quantum annealing in the form of small-gap avoided level crossings characterizing the hard problems. To assess the computational power of the device we compare it against optimized classical algorithms.

Nature Physics 10, 218–224 (2014) doi:10.1038/nphys2900

http://www.nature.com/nphys/journal/v10/n3/full/nphys2900.html

Electrons go ballistic

'Juan José Palacios'

A recent experiment shows that graphene nanoribbons can be grown to be perect conductors where electrons travel long distances without coming across a single obstacle.

Nature Physics 10, 182–183 (2014) doi:10.1038/nphys2909

http://www.nature.com/nphys/journal/v10/n3/full/nphys2909.html#access

Quantum droplets of electrons and holes

'A. E. Almand-Hunter, H. Li, S. T. Cundiff, M. Mootz, M. Kira & S. W. Koch'

Interacting many-body systems are characterized by stable configurations of objects—ranging from elementary particles to cosmological formations1, 2, 3—that also act as building blocks for more complicated structures. It is often possible to incorporate interactions in theoretical treatments of crystalline solids by introducing suitable quasiparticles that have an effective mass, spin or charge4, 5 which in turn affects the material’s conductivity, optical response or phase transitions2, 6, 7. Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator8, superconductivity or the pseudogap phase of high-temperature superconductors9, 10, 11. In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton12, 13, which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons14, and even polyexcitons may exist15, 16. In indirect-gap semiconductors such as germanium or silicon, a thermodynamic phase transition may produce electron–hole droplets whose diameter can approach the micrometre range17, 18. In direct-gap semiconductors such as gallium arsenide, the exciton lifetime is too short for such a thermodynamic process. Instead, different quasiparticle configurations are stabilized dominantly by many-body interactions, not by thermalization. The resulting non-equilibrium quantum kinetics is so complicated that stable aggregates containing three or more Coulomb-correlated electron–hole pairs remain mostly unexplored. Here we study such complex aggregates and identify a new stable configuration of charged particles that we call a quantum droplet. This configuration exists in a plasma and exhibits quantization owing to its small size. It is charge neutral and contains a small number of particles with a pair-correlation function that is characteristic of a liquid. We present experimental and theoretical evidence for the existence of quantum droplets in an electron–hole plasma created in a gallium arsenide quantum well by ultrashort optical pulses.

Nature 506, 471–475 (27 February 2014) doi:10.1038/nature12994

http://www.nature.com/nature/journal/v506/n7489/full/nature12994.html

Febr. 1. - Febr. 21. (2014)

Assorted by: Fülöp Bálint

Nanowire section (InAs, InSb)

Magnetotransport Subband Spectroscopy in InAs Nanowires

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.076801

Controlled Synthesis of Phase-Pure InAs Nanowires on Si(111) by Diminishing the Diameter to 10 nm

http://pubs.acs.org/doi/abs/10.1021/nl4040847

Phase Separation in Single InxGa1–xN Nanowires Revealed through a Hard X-ray Synchrotron Nanoprobe

http://pubs.acs.org/doi/abs/10.1021/nl4042752

Mobility Enhancement by Sb-mediated Minimisation of Stacking Fault Density in InAs Nanowires Grown on Silicon

http://pubs.acs.org/doi/abs/10.1021/nl5001554

Strong Terahertz Emission and Its Origin from Catalyst-Free InAs Nanowire Arrays

http://pubs.acs.org/doi/abs/10.1021/nl404737r

Long Term Stability of Nanowire Nanoelectronics in Physiological Environments

http://pubs.acs.org/doi/abs/10.1021/nl500070h

Graphene section

Electron–Phonon Interactions and the Intrinsic Electrical Resistivity of Graphene

http://pubs.acs.org/doi/abs/10.1021/nl402696q

Graphene Oxide-Based Carbon Interconnecting Layer for Polymer Tandem Solar Cells

http://pubs.acs.org/doi/abs/10.1021/nl4046284

Relaxation of optically excited carriers in graphene: Anomalous diffusion and Lévy flights

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.075414

Quantum corrections to thermopower and conductivity in graphene

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.075411

Quantum phase transitions into Kondo states in bilayer graphene

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.081101

Quantum Search on Graphene Lattices

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.070504

High-Contrast Terahertz Wave Modulation by Gated Graphene Enhanced by Extraordinary Transmission through Ring Apertures

http://pubs.acs.org/doi/abs/10.1021/nl4041274

Transport Measurement of Landau Level Gaps in Bilayer Graphene with Layer Polarization Control

http://pubs.acs.org/doi/abs/10.1021/nl4043399

Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots

http://pubs.acs.org/doi/abs/10.1021/nn500368m

Self-Organized Platinum Nanoparticles on Freestanding Graphene

http://pubs.acs.org/doi/abs/10.1021/nn406394f

Others (MCBJ, AFM, Majorana, QD, etc.)

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

http://pubs.acs.org/doi/abs/10.1021/nl4041687

Hot-Electron Photodetection with a Plasmonic Nanostripe Antenna

http://pubs.acs.org/doi/abs/10.1021/nl4044373

Electronic states of wires and slabs of topological insulators: Quantum Hall effects and edge transport

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.085305

Annihilation of Colliding Bogoliubov Quasiparticles Reveals their Majorana Nature

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.070604

Signatures of Majorana fermions in topological insulator Josephson junction devices

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.075106

Electrochemical Charge-Transfer Resistance in Carbon Nanotube Composites

http://pubs.acs.org/doi/abs/10.1021/nl404349g

Flexible Power Fabrics Made of Carbon Nanotubes for Harvesting Thermoelectricity

http://pubs.acs.org/doi/abs/10.1021/nn405893t

Just for fun

Synthetic Antiferromagnetic Nanoparticles as Potential Contrast Agents in MRI

http://pubs.acs.org/doi/abs/10.1021/nn406158h

Fully Solution-Processed Semitransparent Organic Solar Cells with a Silver Nanowire Cathode and a Conducting Polymer Anode

http://pubs.acs.org/doi/abs/10.1021/nn406672n

Using Room Temperature Current Noise To Characterize Single Molecular Spectra

http://pubs.acs.org/doi/abs/10.1021/nn404526w

Laser Damage Helps the Eavesdropper in Quantum Cryptography

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.070503

A New Type of Porous Graphite Foams and Their Integrated Composites with Oxide/Polymer Core/Shell Nanowires for Supercapacitors: Structural Design, Fabrication, and Full Supercapacitor Demonstrations

http://pubs.acs.org/doi/abs/10.1021/nl5001778

Ultrafast Modulation of the Plasma Frequency of Vertically Aligned Indium Tin Oxide Rods

http://pubs.acs.org/doi/abs/10.1021/nl4028044

A bulk graphene mimic

http://www.nature.com/nature/journal/v506/n7488/full/506269d.html

Quantum information: Strength of weak measurementsQuantum information: Strength of weak measurements

The back-action of a weak measurement on the electron spin of a nitrogen–vacancy centre in diamond can be used to steer the associated nuclear spin towards a desired state.

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2908.html

Not so neutral

Switzerland’s science landscape is under threat after a narrow majority of citizens voted for tighter immigration rules that could restrict the number of foreign scientists who work in the country.

http://www.nature.com/news/not-so-neutral-1.14725

Astronomy: Death of a comet

Before it shattered near the Sun, Comet ISON became a scientific celebrity. Now researchers are trying to piece together its lessons.

http://www.nature.com/news/astronomy-death-of-a-comet-1.14741

Recent interesting articles

From NanoWiki

Contents

Márc. 11. - Márc. 17. (2014)

Febr. 22. - Márc. 3. (2014)

Febr. 1. - Febr. 21. (2014)

Nanowire section (InAs, InSb)

Graphene section

Others (MCBJ, AFM, Majorana, QD, etc.)

Just for fun

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