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Spin-Orbit-Mediated Manipulation of Heavy-Hole Spin Qubits in gated Semiconductor Nanodevices

Physical Review Letters 2012年第10期109卷 107201-107201页

作者： P. Szumniak S. Bednarek B. Partoens F. M. Peeters Faculty of Physics and Applied Computer Science AGH University of Science and Technology al. Mickiewicza 30 30-059 Kraków Poland Departement Fysica Universiteit Antwerpen Groenenborgerlaan 171 B-2020 Antwerpen Belgium

A novel spintronic nanodevice is proposed that is able to manipulate the single heavy-hole spin state in a coherent manner. It can act as a single quantum logic gate. The heavy-hole spin transformations are realized by transporting the hole around closed loops defined by metal gates deposited on top of the nanodevice. The device exploits Dresselhaus spin-orbit interaction, which translates the spatial motion of the hole into a rotation of the spin. The proposed quantum gate operates on subnanosecond time scales and requires only the application of a weak static voltage which allows for addressing heavy-hole spin qubits individually. Our results are supported by quantum mechanical time-dependent calculations within the four-band Luttinger-Kohn model.

关键词： SPIN-orbit interactions (Physics) HOLES (Electron deficiencies) SEMICONDUCTORS gate array circuits SPINTRONICS MATHEMATICAL models QUANTUM theory PHASE transformations (Physics)

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Hole spin relaxation in p-type (111) GaAs quantum wells

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Physical Review B 2012年第23期85卷 235308-235308页

作者： L. Wang M. W. Wu Hefei National Laboratory for Physical Sciences Microscale and Department of Physics University of Science and Technology of China Hefei Anhui 230026 China

Hole spin relaxation in p-type (111) GaAs quantum wells is investigated for cases in which only the lowest hole subband, which is heavy-hole like in (111) GaAs/AlAs and light-hole like in (111) GaAs/InP quantum wells, is relevant. The subband Löwdin perturbation method is applied to obtain the effective Hamiltonian including the Dresselhaus and Rashba spin-orbit couplings. Under a proper gate voltage, the total in-plane effective magnetic field in (111) GaAs/AlAs quantum wells can be strongly suppressed in the entire momentum space, while that in (111) GaAs/InP quantum wells can be suppressed only on a special momentum circle. The hole spin relaxation due to the D’yakonov-Perel’ and Elliott-Yafet mechanisms is calculated by means of the fully microscopic kinetic spin Bloch equation approach with all the relevant scatterings explicitly included. For (111) GaAs/AlAs quantum wells, extremely long heavy-hole spin relaxation time (up to hundreds of nanoseconds) is predicted. In particular, we predict a pronounced peak in the gate-voltage dependence of the heavy-hole spin relaxation time due to the D’yakonov-Perel’ mechanism. This peak originates from the suppression of the unique inhomogeneous broadening in (111) GaAs/AlAs quantum wells. Moreover, the Elliott-Yafet mechanism influences the spin relaxation only around the peak area. In (111) GaAs/InP quantum wells, a mild peak, similar to the case for electrons in (111) GaAs quantum wells, is also predicted by the gate-voltage dependence of the light-hole spin relaxation time. The contribution of the Elliott-Yafet mechanism is found to always be negligible in this case.

关键词： HOLES (Electron deficiencies) NUCLEAR spin RELAXATION phenomena GALLIUM arsenide MAGNETIC fields QUANTUM wells ARSENIDES gate array circuits

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Coherent electrical rotations of valley states in Si quantum dots using the phase of the valley-orbit coupling

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Physical Review B 2012年第3期86卷 035321-035321页

作者： Yue Wu Dimitrie Culcer ICQD Hefei National Laboratory for Physical Sciences at the Microscale University of Science and Technology of China Hefei Anhui 230026 China

A gate electric field has a small but non-negligible effect on the phase of the valley-orbit coupling in Si quantum dots. Finite interdot tunneling between valley eigenstates in a double quantum dot is enabled by a small difference in the phase of the valley-orbit coupling between the two dots, and it in turn allows controllable rotations of two-dot valley eigenstates at a level anticrossing. We present a comprehensive analytical discussion of this process, with estimates for realistic structures.

关键词： COHERENT states QUANTUM dots SILICON ELECTRIC fields gate array circuits ENERGY levels (Quantum mechanics)

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Band gaps in graphene via periodic electrostatic gating

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Physical Review B 2012年第23期85卷 235432-235432页

作者： Jesper Goor Pedersen Thomas Garm Pedersen Department of Physics and Nanotechnology Aalborg University Skjernvej 4A DK-9220 Aalborg East Denmark Center for Nanostructured Graphene (CNG) Aalborg University DK-9220 Aalborg East Denmark

Much attention has been focused on ways of rendering graphene semiconducting. We study periodically gated graphene in a tight-binding model and find that, contrary to predictions based on the Dirac equation, it is possible to open a band gap at the Fermi level using electrostatic gating of graphene. However, comparing to other methods of periodically modulating graphene, namely, perforated graphene structures, we find that the resulting band gap is significantly smaller. We discuss the intricate dependence of the band gap on the magnitude of the gate potential as well as the exact geometry of the edge of the gate region. The role of the overlap of the eigenstates with the gate region is elucidated. Considering more realistic gate potentials, we find that introducing smoothing in the potential distribution, even over a range of little more than a single carbon atom, reduces the attainable band gap significantly. This represents a serious challenge to achieving gapped graphene via periodic gating.

关键词： ENERGY gaps (Physics) GRAPHENE ELECTROSTATICS DIFFRACTION gratings SEMICONDUCTORS gate array circuits ENERGY levels (Quantum mechanics)

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Polaron effects on the dc- and ac-tunneling characteristics of molecular Josephson junctions

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Physical Review B 2012年第3期86卷 035406-035406页

作者： B. H. Wu J. C. Cao C. Timm Department of Applied Physics Donghua University 2999 North Renmin Road Shanghai 201620 China Key Laboratory of Terahertz Solid-State Technology Shanghai Institute of Microsystem and Information Technology Chinese Academy of Sciences 865 Changning Road Shanghai 200050 China Institute of Theoretical Physics Technische Universität Dresden 01062 Dresden Germany

We study the interplay of polaronic effect and superconductivity in transport through molecular Josephson junctions. The tunneling rates of electrons are dominated by vibronic replicas of the superconducting gap, which show up as prominent features in the differential conductance for the dc and ac current. For relatively large molecule-lead coupling, a features that appears when the Josephson frequency matches the vibron frequency can be identified with an over-the-gap structure observed by Marchenkov et al. [Nat. Nanotech. 2, 481 (2007)]. However, we are more concerned with the weak-coupling limit, where resonant tunneling through the molecular level dominates. We find that certain features involving both Andreev reflection and vibron emission show an unusual shift of the bias voltage V at their maximum with the gate voltage Vg as V∼(2/3)Vg. Moreover, due to the polaronic effect, the ac Josephson current shows a phase shift of π when the bias eV is increased by one vibronic energy quantum ℏωv. This distinctive even-odd effect is explained in terms of the different sign of the coupling to vibrons of electrons and of Andreev-reflected holes.

关键词： TUNNELING (Physics) POLARONS MOLECULAR electronics JOSEPHSON junctions SUPERCONDUCTIVITY ELECTRON transport gate array circuits

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Kondo temperature of a quantum dot

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Physical Review B 2012年第23期85卷 235311-235311页

作者： Seungjoo Nah Michael Pustilnik School of Physics Georgia Institute of Technology Atlanta Georgia 30332 USA

We study the dependence of the Kondo temperature on the gate voltage in a strongly blockaded quantum dot with a small single-particle level spacing. We show that the dependence cannot be fitted to that of the Anderson impurity model with the gate voltage-independent level width. The effect originates in high-order tunneling processes, which make a dominant contribution to the exchange amplitude when the gate voltage is tuned away from the middle of the Coulomb blockade valley.

关键词： KONDO effect QUANTUM dots gate array circuits ANDERSON model TUNNELING (Physics) COULOMB potential

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Water-assisted mobile charge induced screening and origin of hysteresis in carbon nanotube field-effect transistors

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Physical Review B 2012年第11期86卷 115444-115444页

作者： Y. Pascal-Levy E. Shifman M. Pal-Chowdhury I. Kalifa T. Rabkin O. Shtempluck A. Razin V. Kochetkov Y. E. Yaish Russell Berrie Nanotechnology Institute Technion Haifa 32000 Israel Department of Electrical Engineering Technion Haifa 32000 Israel

Carbon nanotube field-effect transistors (CNT FETs) have many possible applications in future nanoelectronics due to their excellent properties. However, one of the major challenges regarding their performance is the noticeable gate hysteresis which is often displayed in their transfer characteristics. The hysteresis phenomenon is often attributed to water-mediated charge transfer between the CNT and the dielectric layer or the CNT and the water layer itself. In this study, we implement the usage of current versus time measurements in addition to the traditional transfer characteristics to accurately extract the time constants of the hysteresis of suspended and on-surface CNT FETs. Following a thorough study, we provide experimental evidence that the hysteresis phenomenon of suspended CNT FETs, as well as of on-surface CNT FETs which operate at low gate voltage regimes (|Vg|<3 V), is based on gate-induced, water-assisted redistribution of mobile charge on the SiO2 surface, and is not related to charge injection from the CNT itself. Our model is confirmed by an electronic-force-microscopy-based measurement technique which enables us to quantify the temporal surface charge distribution while measuring CNT currents.

关键词： ELECTRIC charge & distribution ELECTRON mobility HYSTERESIS CARBON nanotubes FIELD-effect transistors NANOELECTRONICS gate array circuits

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Ionic screening of charged impurities in electrolytically gated graphene

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Physical Review B 2012年第11期86卷 115437-115437页

作者： Z. L. Mišković P. Sharma F. O. Goodman Department of Applied Mathematics University of Waterloo Waterloo Ontario Canada N2L 3G1

We present a model for dual-gated, single-layer graphene, with a back gate separated by a layer of oxide, and the top gate potential applied through a thick layer of liquid electrolyte that contains mobile ions in a diffuse layer, described in the Debye-Hückel approximation, which is separated from graphene by a charge-free Stern layer. After deriving a nonlinear equation for the average charge carrier density in graphene in terms of the gate potentials, we use the Green's function of the Poisson equation to express the fluctuating part of the electrostatic potential in the plane of graphene in terms of the distribution function for fixed charged impurities in the oxide. By using both the Thomas-Fermi and the random phase approximations of graphene's response at nonzero temperature, we show that the presence of mobile ions in the electrolyte significantly increases graphene's screening ability of the in-plane potential for a single impurity, accentuates Friedel oscillations in that potential, and gives rise to a linear plasmon dispersion in doped graphene at long wavelengths. In the case of multiple charged impurities in the oxide, the increasing ion concentration in the electrolyte causes a reduction in the autocorrelation function of the fluctuating in-plane potential when the impurities are uncorrelated. However, when the impurities are correlated, the relative effect of the increased ion concentration in the electrolyte is drastically reduced, while the autocorrelation function in this case takes negative values in a range of interimpurity distances.

关键词： POINT defects ELECTROLYSIS GRAPHENE gate array circuits APPROXIMATION theory NONLINEAR differential equations AUTOCORRELATION (Statistics)

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Configuration interaction calculations of the controlled phase gate in double quantum dot qubits

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Physical Review B 2012年第3期85卷 035319-035319页

作者： Erik Nielsen Richard P. Muller Malcolm S. Carroll Sandia National Laboratories Albuquerque New Mexico 87185 USA

We consider qubit coupling resulting from the capacitive coupling between two double quantum dot (DQD) singlet-triplet qubits. Calculations of the coupling when the two DQDs are detuned symmetrically or asymmetrically are performed using a full configuration interaction (CI). The full CI reveals behavior that is not observed by more commonly used approximations such as Heitler London or Hund Mulliken, particularly related to the operation of both DQDs in the (0,2) charge sector. We find that there are multiple points in detuning space where a two-qubit entangling gate can be realized, and that tradeoffs between coupling magnitude and sensitivity to fluctuations in detuning make a case for operating the gate in the (0,2) regime not commonly considered.

关键词： gate array circuits QUANTUM dots OBSERVATION (Scientific method) APPROXIMATION theory FLUCTUATIONS (Physics) QUANTUM electronics

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Hybrid topological-spin qubit systems for two-qubit-spin gates

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Physical Review B 2012年第10期86卷 104511-104511页

作者： Martin Leijnse Karsten Flensberg Center for Quantum Devices Niels Bohr Institute University of Copenhagen 2100 Copenhagen Ø Denmark

We investigate a hybrid quantum system involving spin qubits, based on the spins of electrons confined in quantum dots, and topological qubits, based on Majorana fermions. In such a system, gated control of the charge on the quantum dots allows transfer of quantum information between the spin and topological qubits, and the topological system can be used to facilitate transfer of spin qubits between spatially separated quantum dots and to initialize entangled spin-qubit pairs. Here, we show that the coupling to the topological system also makes it possible to perform entangling two-qubit gates on spatially separated spin qubits. The two-qubit gates are based on a combination of topologically protected braiding operations, gate-controlled charge transfer between the dots and edge Majorana modes, and measurements of the state of the topological qubits.

关键词： NUCLEAR spin QUBITS gate array circuits QUANTUM theory QUANTUM dots QUANTUM entanglement CHARGE transfer

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