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ABSTRACT: Considering screeening of electron scattering interactions in terms of the
finite-temperature STLS theory and solving the linearized Boltzmann equation
(with no appeal to a relaxation time approximation), we present a theoretical
analysis of the low-temperature Seebeck effect in two-dimensional
semiconductors with dilute electron densities. We find that the temperature
($T$) dependencies of the diffusion and phonon-drag thermoelectric powers
($S_d$ and $S_g$) can no longer be described by the conventional simple
power-laws. As temperature increases, $|S_d|/T$ decreases when $T\gtrsim 0.1
\epsilon_F$ ($\epsilon_F$ is the Fermi energy), while $|S_g|$ first increases
and then falls, resulting a peak located at a temperature between
Bloch-Gr\"uneisen temperature and $\epsilon_F$.
06/2011;
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ABSTRACT: We present a kinetic equation approach to investigate dc transport properties of graphene in the diffusive regime considering long-range electron-impurity scattering. In our study, the effects of interband correlation (or polarization) on conductivity are taken into account. We find that the conductivity contains not only the usual term inversely proportional to impurity density N<sub>i</sub> but also an anomalous term that is linear in N<sub>i</sub> . This leads to a minimum in the density dependence of conductivity when the electron density N<sub>e</sub> is equal to a finite critical value N<sub>c</sub> . The effects of various scattering potentials on the conductivity minimum are also analyzed. Using typical experimental parameters, we find that for random-phase-approximation–screened electron-impurity scattering, the minimum conductivity is about 4.42e<sup>2</sup>/h when N<sub>e</sub>≈0.11N<sub>i</sub> , and the conductivity varies almost linearly with the electron density for N<sub>e</sub>≫N<sub>i</sub> .
Journal of Applied Physics 09/2008; · 2.17 Impact Factor
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ABSTRACT: We present a kinetic equation approach to investigate the anomalous Hall effect in two-dimensional systems with magnetization considering both electron-impurity and electron-phonon scatterings. In our study, the spin-orbit interaction due to the external driving electric field as well as the extrinsic spin-orbit couplings induced by electron-impurity and electron-phonon scatterings are taken into account, while the intrinsic Rashba and Dresselhaus spin-orbit couplings are ignored. We derive the side-jump contributions to anomalous Hall current in terms of the distribution function and obtain the skew-scattering contribution by considering electron-impurity (and electron-phonon) scattering up to the second Born approximation. By performing a numerical calculation for InSb-based quantum wells, the temperature dependencies of the various components of anomalous Hall current are examined. We also discuss the roles of electron-impurity and electron-phonon scatterings in contributing to the total anomalous Hall current.
Phys. Rev. B. 11/2007; 76(19).
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ABSTRACT: We analyze inelastic cotunneling through an interacting quantum dot subject to an ambient magnetic field in the weak tunneling regime under a non-adiabatic time-dependent bias-voltage. Our results clearly exhibit photon-assisted satellites and an overall suppression of differential conductance with increasing driving amplitude, which is consistent with experiments. We also predict a zero-anomaly in differential conductance under an appropriate driving frequency. Comment: Phys. Lett. A (in press)
08/2007;
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ABSTRACT: The spin-Hall effect in semiconductors with spin-orbit coupling has been extensively investigated recently. However, such spin-Hall current was shown to vanish quite generally in the diffusive regime in infinitely large two-dimensional (2D) semiconductors with Rashba spin-orbit interaction. Here, employing a kinetic equation approach, we investigate the spin-Hall effect in finite-size Rashba 2D semiconductors. We find that the fluctuation of electron density near the sample boundaries may give rise to a nonvanishing spin polarization, notwithstanding the vanishing of spin-Hall current. With decreasing impurity density, the spin polarization increases. Our result indicates that the spin-Hall current does not directly relate to spin accumulation, the latter of which may be observed experimentally. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 02/2007; 4(2):336 - 339.
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ABSTRACT: We employ a helicity-basis kinetic equation approach to investigate the anomalous Hall effect in two-dimensional narrow-band semiconductors considering both Rashba and extrinsic spin-orbit (SO) couplings, as well as a SO coupling directly induced by an external driving electric field. Taking account of long-range electron-impurity scattering up to the second Born approximation, we find that the various components of the anomalous Hall current fit into two classes: (a) side-jump and (b) skew scattering anomalous Hall currents. The side-jump anomalous Hall current involves contributions not only from the extrinsic SO coupling but also from the SO coupling due to the driving electric field. It also contains a component which arises from the Rashba SO coupling and relates to the off-diagonal elements of the helicity-basis distribution function. The skew scattering anomalous Hall effect arises from the anisotropy of the diagonal elements of the distribution function and it is a result of both the Rashba and extrinsic SO interactions. Further, we perform a numerical calculation to study the anomalous Hall effect in a typical InSb/AlInSb quantum well. The dependencies of the side-jump and skew scattering anomalous Hall conductivities on magnetization and on the Rashba SO coupling constant are examined. Comment: 16 pages, 4 figures, accepted for publication in PRB
09/2006;
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ABSTRACT: Motivated by a recent experiment[Nature {\bf 442}, 176 (2006)], we present a quantitative microscopic theory to investigate the inverse spin-Hall effect with spin injection into aluminum considering both intrinsic and extrinsic spin-orbit couplings using the orthogonalized-plane-wave method. Our theoretical results are in good agreement with the experimental data. It is also clear that the magnitude of the anomalous Hall resistivity is mainly due to contributions from extrinsic skew scattering, while its spatial variation is determined by the intrinsic spin-orbit coupling. Comment: 5 pages, 3 figures
08/2006;
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ABSTRACT: Employing a nonequilibrium Green's function approach, we examine the effects of long-range hole-impurity scattering on spin-Hall current in $p$-type bulk semiconductors within the framework of the self-consistent Born approximation. We find that, contrary to the null effect of short-range scattering on spin-Hall current, long-range collisions do produce a nonvanishing contribution to the spin-Hall current, which is independent of impurity density in the diffusive regime and relates only to hole states near the Fermi surface. The sign of this contribution is opposite to that of the previously predicted disorder-independent spin-Hall current, leading to a sign change of the total spin-Hall current as hole density varies. Furthermore, we also make clear that the disorder-independent spin-Hall effect is a result of an interband polarization directly induced by the dc electric field with contributions from all hole states in the Fermi sea. Comment: 9 pages, 1 figure
12/2005;
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ABSTRACT: We employ a quantum Langevin equation approach to establish non-Markovian dynamical equations, on a fully microscopic basis, to investigate the measurement of the state of a coupled quantum dot qubit by a nearby quantum point contact. The ensuing Bloch equations allow us to examine qubit relaxation and decoherence induced by measurement, and also the noise spectrum of meter output current with the help of a quantum regression theorem, at arbitrary bias-voltage and temperature. Our analyses provide a clear resolution of a recent debate concerning the occurrence of a quantum oscillation peak in the noise spectrum. Comment: 5 pages, 3 figures, submitted, published version in Phys. Rev. B
11/2005;
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ABSTRACT: We present a quantum Boltzmann equation analysis of the spin-Hall effect in a diffusive Rashba two-dimensional electron system. Within the framework of the self-consistent Born approximation, we consider the roles of disorder-induced quasiclassical relaxation, collisional broadening of the quasiparticles, and the intracollisional field effect in regard to spin-Hall dynamics. We present an analytical proof that the spin-Hall current vanishes, independently of the coupling strength, of the quasiparticle broadening, of temperature and of the specific form of the isotropic scattering potential. A sum relation of the collision terms in a helicity basis is also examined.
10/2005;
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ABSTRACT: We explore inelastic cotunneling through a strongly Coulomb-blockaded quantum dot attached to two ferromagnetic leads in the weak coupling limit using a generic quantum Langevin equation approach. We first develop a Bloch-type equation microscopically to describe the cotunneling-induced spin relaxation dynamics, and then develop explicit analytical expressions for the local magnetization, current, and its fluctuations. On this basis, we predict a novel zero-bias anomaly of the differential conductance in the absence of a magnetic field for the anti-parallel configuration, and asymmetric peak splitting in a magnetic field. Also, for the same system with large polarization, we find a negative zero-frequency differential shot noise in the low positive bias-voltage region. All these effects are ascribed to rapid spin-reversal due to underlying spin-flip cotunneling.
10/2005;
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ABSTRACT: We present a theoretical analysis of several aspects of nonequilibirum cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a finite magnetic field in the weak coupling limit. We carry this out by developing a generic quantum Heisenberg-Langevin equation approach leading to a set of Bloch dynamical equations which describe the nonequilibrium cotunneling in a convenient and compact way. These equations describe the time evolution of the spin variables of the QD explicitly in terms of the response and correlation functions of the free reservoir variables. This scheme not only provides analytical expressions for the relaxation and decoherence of the localized spin induced by cotunneling, but it also facilitates evaluations of the nonequilibrium magnetization, the charge current, and the spin current at arbitrary bias-voltage, magnetic field, and temperature. We find that all cotunneling events produce decoherence, but relaxation stems only from {\em inelastic} spin-flip cotunneling processes. Moreover, our specific calculations show that cotunneling processes involving electron transfer (both spin-flip and non-spin-flip) contribute to charge current, while spin-flip cotunneling processes are required to produce a net spin current in the asymmetric coupling case. We also point out that under the influence of a nonzero magnetic field, spin-flip cotunneling is an energy-consuming process requiring a sufficiently strong external bias-voltage for activation, explaining the behavior of differential conductance at low temperature: in particular, the splitting of the zero-bias anomaly in the charge current and a broad zero-magnitude "window" of differential conductance for the spin current near zero-bias-voltage. Comment: 15 pages, 5 figures, published version, to appear in Phys. Rev. B
08/2005;
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ABSTRACT: We present a theoretical analysis of the effect of inelastic electron scattering on current and its fluctuations in a mesoscopic quantum dot (QD) connected to two leads, based on a recently developed nonperturbative technique involving the approximate mapping of the many-body electron-phonon coupling problem onto a multichannel single-electron scattering problem. In this, we apply the B\"uttiker scattering theory of shot noise for a two-terminal mesoscopic device to the multichannel case with differing weight factors and examine zero-frequency shot noise for two special cases: (i) a single-molecule QD and (ii) coupled semiconductor QDs. The nonequilibrium Green's function method facilitates calculation of single-electron transmission and reflection amplitudes for inelastic processes under nonequilibrium conditions in the mapping model. For the single-molecule QD we find that, in the presence of the electron-phonon interaction, both differential conductance and differential shot noise display additional peaks as bias-voltage increases due to phonon-assisted processes. In the case of coupled QDs, our nonperturbative calculations account for the electron-phonon interaction on an equal footing with couplings to the leads, as well as the coupling between the two dots. Our results exhibit oscillations in both the current and shot noise as functions of the energy difference between the two QDs, resulting from the spontaneous emission of phonons in the nonlinear transport process. In the "zero-phonon" resonant tunneling regime, the shot noise exhibits a double peak, while in the "one-phonon" region, only a single peak appears. Comment: 10 pages, 6 figures, some minor changes, accepted by Phys. Rev. B
07/2004;
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ABSTRACT: We analyze electron transport through a biased asymmetric double-dot system in a parallel arrangement between leads. We show that the imposition of a dot-to-dot bias induces changes in the symmetry of the wave functions associated with the double-dot energy levels, resulting in a dip in the dependence of the lead-to-lead current on dot-to-dot bias. We also demonstrate that when the dot-to-dot bias compensates (energetically) the structure asymmetry, the lead-to-lead current will be an oscillating function of an applied magnetic field in the case in which only one of the double-dot levels is conductive. However, destructive interference between different parts of the same dot gives rise to suppression of current at high magnetic fields. © 2004 American Institute of Physics.
Journal of Applied Physics 03/2004; 95(7):3557-3560. · 2.17 Impact Factor
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ABSTRACT: We present a theory accounting for the origin of high-frequency current oscillations in a double barrier quantum-well system. The origin of such current oscillations is traced to the development of a dynamic emitter quantum-well and the concomitant coupling of the energy levels in the double barrier quantum-well system. The relationship between the oscillation frequency and the energy level structure of the system is expressed as ν = E 0 /h: A self-consistent, time-dependent Wigner–Poisson numerical computer experiment is used to exhibit remarkable intrinsic, sustained current oscillations in the double-barrier quantum well at terahertz frequencies; and a procedure for calculating E 0 , the energy difference at time t 0 (defined such that the contribution to the energy difference from the potential oscillation is zero) is also presented. The simulated oscillation frequency determined using the Wigner–Poisson analysis is in very good agreement with that calculated using a Schrödinger equation with a self-consistent potential determined from the Poisson equation. 2003 Elsevier Science B.V. All rights reserved.
Physics Letters A. 01/2003; 3112080(79).
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ABSTRACT: We examine the high-frequency differential conductivity response properties of semiconductor superlattices having various miniband dispersion laws. Our analysis shows that the anharmonicity of Bloch oscillations (beyond tight-binding approximation) leads to the occurrence of negative high-frequency differential conductivity at frequency multiples of the Bloch frequency. This effect can arise even in regions of positive static differential conductivity. The influence of strong electron scattering by optic phonons is analyzed. We propose an optimal superlattice miniband dispersion law to achieve high-frequency field amplification.
09/2002;
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ABSTRACT: We present a microscopic analysis of electron spin dynamics in the presence of an external magnetic field for non-centrosymmetric semiconductors in which the D'yakonov-Perel' spin-orbit interaction is the dominant spin relaxation mechanism. We implement a fully microscopic two-step calculation, in which the relaxation of orbital motion due to electron-bath coupling is the first step and spin relaxation due to spin-orbit coupling is the second step. On this basis, we derive a set of Bloch equations for spin with the relaxation times T_1 and T_2 obtained microscopically. We show that in bulk semiconductors without magnetic field, T_1 = T_2, whereas for a quantum well with a magnetic field applied along the growth direction T_1 = T_2/2 for any magnetic field strength.
09/2002;
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ABSTRACT: A fully microscopic theory of electron spin relaxation by the D'yakonov-Perel' type spin-orbit coupling is developed for a semiconductor quantum well with a magnetic field applied in the growth direction of the well. We derive the Bloch equations for an electron spin in the well and define microscopic expressions for the spin relaxation times. The dependencies of the electron spin relaxation rate on the lowest quantum well subband energy, magnetic field and temperature are analyzed. Comment: Revised version as will appear in Physical Review B
09/2002;
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ABSTRACT: We analyze the dynamics of a nanomechanical oscillator coupled to an electrical tunnel junction with an arbitrary voltage applied to the junction and arbitrary temperature of electrons in leads. We obtain the explicit expressions for the fluctuations of oscillator position, its damping/decoherence rate, and the current through the structure. It is shown that quantum heating of the oscillator results in nonlinearity of the current-voltage characteristics. The effects of mechanical vacuum fluctuations are also discussed.
09/2002;
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ABSTRACT: We examine the transport and microwave properties of the tunnel-coupled double-dot structure with parallel arrangement between leads in the presence of Aharonov-Bohm magnetic flux. The nonequilibrium Green’s function formalism is employed with account of electron-phonon coupling and interdot Coulomb repulsion. We obtain the energies, populations, and linewidths of the bonding and antibonding states in the double-dot structure as well as its current-voltage characteristics and the bias dependence of the electromagnetic energy absorbed by the system. For resonant magnetic fluxes when only one level is connected to the leads, the current-voltage characteristics have only one step, whereas for nonresonant fluxes there are either two steps or one sharp step depending on the common equilibrium chemical potential of the leads. In the case of two steps, the current value in the plateau region between the steps is the oscillatory function of applied magnetic flux. It is shown that the disconnection of one of the levels from the leads at the resonant phases gives rise to the residual absorption of the external electromagnetic field at high bias which vanishes otherwise.
Phys. Rev. B. 08/2002; 66(8).
