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S. Y. Liu

Publications and source records attributed to S. Y. Liu.

34 records · Page 2Linked to original sources
arXiv2006

Vanishing spin-Hall current in a diffusive Rashba two-dimensional electron system: A quantum Boltzmann equation approach

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.

cond-mat.mes-hall↗
arXiv2005

A kinetic equation approach to the anomalous Hall effect in a diffusive Rashba two-dimensional electron system with magnetization

We present a two-band kinetic equation method to investigate the anomalous Hall effect in a Rashba two-dimensional electron system subjected to a homogeneous magnetization. The electron-impurity scattering is taken into account in the self-consistent Born approximation. It is demonstrated that the impurity-density-free anomalous Hall conductivity arises from an intrinsic and a disorder-mediated mechanisms, associated respectively with the electron states under and near the Fermi surface. The intrinsic mechanism relates to a dc-field-induced transition process, or in other words, a linear stationary Rabi process. The disorder-mediated one corresponds to a scattering between impurities and the electrons participating in longitudinal transport. Numerically, the dependencies of the anomalous Hall conductivity on the spin-orbit coupling constants and the strength of the magnetization are demonstrated for both the short- and long-range collisions.

cond-mat.mes-hall↗
arXiv2005

Disorder effects on the spin-Hall current in a diffusive Rashba two-dimensional heavy-hole system

We investigate the spin-Hall effect in a two-dimensional heavy-hole system with Rashba spin-orbit coupling using a nonequilibrium Green's function approach. Both the short- and long-range disorder scatterings are considered in the self-consistent Born approximation. We find that, in the case of long-range collisions, the disorder-mediated process leads to an enhancement of the spin-Hall current at high heavy-hole density, whereas for short-range scatterings it gives a vanishing contribution. This result suggests that the recently observed spin-Hall effect in experiment is a result of the sum of the intrinsic and disorder-mediated contributions. We have also calculated the temperature dependence of spin-Hall conductivity, which reveals a decrease with increasing the temperature.

cond-mat.mtrl-sci↗
arXiv2005

Radiation-induced magnetotransport in high-mobility two-dimensional systems: Role of electron heating

Effects of microwave radiation on magnetoresistance are analyzed in a balance-equation scheme that covers regimes of inter- and intra-Landau level processes and takes account of photon-asissted electron transitions as well as radiation-induced change of the electron distribution for high mobility two-dimensional systems. Short-range scatterings due to background impurities and defects are shown to be the dominant direct contributors to the photoresistance oscillations. The electron temperature characterizing the system heating due to irradiation, is derived by balancing the energy absorption from the radiation field and the energy dissipation to the lattice through realistic electron-phonon couplings, exhibiting resonant oscillation. Microwave modulations of Shubnikov de Haas oscillation amplitude are produced together with microwave-induced resistance oscillations, in agreement with experimental findings. In addition, the suppression of the magnetoresistance caused by low-frequency radiation in the higher magnetic field side is also demonstrated.

cond-mat.mes-hall↗
arXiv2005

Microwave modulation of electron temperature and Shubnikov-de Haas oscillation in two-dimensional electron systems

Recently discovered modulations of Shubnikov-de Haas oscillations in microwave-irradiated two-dimensional electron systems are shown to arise from electron heating induced by the radiation. The electron temperature, obtained by balancing the energy absorption from the microwave field and the energy dissipation to the lattice through realistic electron-phonon couplings, exhibits resonance. The modulation of the Shubnikov de Haas oscillation and the suppression of magnetoresistance are demonstrated together with microwave-induced resistance oscillation, in agreement with experimental findings.

cond-mat.mes-hall↗
arXiv2005

The inelastic relaxation time due to electron-electron collisions in high-mobility two-dimensional systems under microwave radiations

In some theoretical analyses of microwave-induced magnetoresistance oscillations in high-mobility two-dimensional systems, the "inelastic relaxation time" $τ_{in}$ due to electron-electron scattering is evaluated using an equilibrium distribution function $f^0$ in the absence of radiation, and it is concluded that $τ_{in}$ is much larger than $τ_{q}$, the single-particle relaxation time due to impurity scattering. However, under the irradiation of a microwave capable of producing magnetoresistance oscillation, the distribution function of the high-mobility electron gas deviates remarkably from $f^0$ at low temperatures. Estimating $τ_{in}$ using an approximate nonequilibrium distribution function rather than using $f^0$, one will find the system to be in the opposite limit $1/τ_{in}\ll 1/τ_{q}$ even for T=0 K. Therefore, models which depend on the assumption $1/τ_{in}\gg 1/τ_{q}$ may not be justifiable.

cond-mat.mes-hall↗
arXiv2005

Spin Hall Effect in a Diffusive Rashba Two-dimensional Electron Gas

A nonequilibrium Green's functions approach to spin-Hall effect is developed in a diffusive two-dimensional electron system with Rashba spin-orbit interaction. In the presence of long-range impurities, the coupled quantum kinetic equations are solved analytically in the self-consistent Born approximation. It is shown that the intrinsic spin-Hall effect stems from the dc-field-induced perturbation of the density of states. In addition, there is an additional disorder-mediated process, which involves the transition of nonequilibrium electrons between two spin-orbit-coupled bands. It results in an additional collision-independent spin-Hall conductivity and leads to the vanishing of the total spin-Hall current even at nonzero temperature.

cond-mat.mtrl-sci↗
arXiv2005

Vanishing of the Dissipationless Spin Hall Effect in a Diffusive Two-Dimensional Electron Gas with Spin-Orbit Coupling

We propose a nonequilibrium Green's function approach to study the spin-Hall effect in a two-dimensional electron system with both the Rashba and Dresselhaus spin-orbit couplings. By taking into account the long-range electron-impurity scattering, the derived kinetic equations are solved numerically. It is found the vanishing of the total zero-temperature dissipationless spin-Hall effect, contributing from the intrinsic and disorder-mediated processes. This result has been examined in the wide ranges of spin-orbit coupling constants and electron density.

cond-mat.mtrl-sci↗
arXiv2004

Mapping the Outflow from G5.89-0.39 in SiO(5-4)

We have mapped the ultracompact HII region, G5.89-0.39, and its molecular surroundings with the Submillimeter Array at 2".8 x 1".8 angular resolution in 1.3 mm continuum, SiO(5-4), and eight other molecular lines. We have resolved for the first time the highly energetic molecular outflow in this region. At this resolution, the outflow is definitely bipolar and appears to originate in a 1.3 mm continuum source. The continuum source peaks in the center of the HII region. The axis of the outflow lines up with a recently discovered O5V star.

astro-ph↗
arXiv2004

Low-temperature transport through a quantum dot between two superconductor leads

We consider a quantum dot coupled to two BCS superconductors with same gap energies $Δ$. The transport properties are investigated by means of infinite-$U$ noncrossing approximation. In equilibrium density of states, Kondo effect shows up as two sharp peaks around the gap bounds. Application of a finite voltage bias leads these peaks to split, leaving suppressed peaks near the edges of energy gap of each lead. The clearest signatures of the Kondo effect in transport are three peaks in the nonlinear differential conductance: one around zero bias, another two at biases $\pm 2Δ$. This result is consistent with recent experiment. We also predict that with decreasing temperature, the differential conductances at biases $\pm 2Δ$ anomalously increase, while the linear conductance descends.

cond-mat.supr-con↗
arXiv2003

Radiation-induced magnetoresistance oscillation in a two-dimensional electron gas in Faraday geometry

Microwave-radiation induced giant magnetoresistance oscillations recently discovered in high-mobility two-dimensional electron systems in a magnetic field, are analyzed theoretically. Multiphoton-assisted impurity scatterings are shown to be the primary origin of the oscillation. Based on a model which considers the interaction of electrons with the electromagnetic fields in Faraday geometry, we are able not only to reproduce the correct period, phase and the negative resistivity of the main oscillation, but also to obtain secondary peaks and additional maxima and minima in the resistivity curve, some of which were already observed in the experiments.

cond-mat.mes-hall↗
arXiv2003

Theory of nonlinear cyclotron resonance in quasi-two-dimensional electron systems

Momentum and energy balance equations are developed for steady-state electron transport and optical absorption under the influence of a dc electric field, an intense ac electric field of terahertz (THz) frequency in a two-dimensional (2D) semiconductor in the presence of a strong magnetic field perpendicular to the 2D plane. These equations are applied to study the intensity-dependent cyclotron resonance (CR) in far-infrared transmission and THz-radiation-induced photoconductivity of GaAs heterostructures in Faraday geometry. We find that the CR peaks and line shapes of the transmittance exhibit different intensity dependence when the intensity of THz field increases in the range above or below a certain critical value. The CR in photoresistivity, however, always enhances with increasing the intensity of the THz field. These results qualitatively agree with the experimental observations. We have clarified that the CR in photoconductivity is not only the result of the electron heating, but also comes from photon-assisted scattering enhancement, especially at high temperatures. The effects of an intense THz field on Faraday angle and ellipticity of magnetically-biased 2D semiconductors have also been demonstrated.

cond-mat.mtrl-sci↗
arXiv2003

Kondo-type transport through an interacting quantum dot coupled to ferromagnetic leads

We investigate the equilibrium and out-of-equilibrium Kondo effects in a single-level interacting quantum dot connected to two ferromagnetic leads. Within the non-crossing approximation, we calculate the total density of states (DOS), the linear conductance, and the nonlinear differential conductance for both the parallel and the anti-parallel alignments of the spin polarization orientation in the leads, followed by a brief discussion regarding the validity of this approach. Numerical calculations show that for the anti-parallel alignment, a single Kondo peak always appears in the equilibrium DOS, resulting in the conventional temperature behavior in the linear conductance and the zero-bias maximum in the differential conductance. The strength of the DOS peak is gradually suppressed with increasing polarization, due to the fact that formation of the Kondo-correlated state is more difficult in the presence of higher polarization. On the contrary, for the parallel configuration the Kondo peak in the DOS descends precipitately and splits into two peaks to form a very steep valley between them. This splitting contributes to the appearance of a "hump" in the temperature-dependent linear conductance and a nonzero-bias maximum in the differential conductance. Moreover, application of a bias voltage can split each Kondo peak into two in the nonequilibrium DOS for both configurations. Finally we point out that the tunnel magnetoresistance could be an effective tool to demonstrate the different Kondo effects in different spin configurations found here.

cond-mat.mes-hall↗
arXiv2002

Elementary operations for quantum logic with a single trapped two-level cold ion beyond Lamb-Dicke limit

A simple alternative scheme for implementing quantum gates with a single trapped cold two-level ion beyond the Lamb-Dicke (LD) limit is proposed. Basing on the quantum dynamics for the laser-ion interaction described by a generalized Jaynes-Cummings model, one can introduce two kinds of elementary quantum operations i.e., the simple rotation on the bare atomic state, generated by applying a resonant pulse, and the joint operation on the internal and external degrees of the ion, performed by using an off-resonant pulse. Several typical quantum gates, including Hadamard gate, controlled-Z and controlled-NOT gates $etc.$, can thus be implemented exactly by using these elementary operations. The experimental parameters including the LD parameter and the durations of the applied laser pulses, for these implementation are derived analytically and numerically. Neither the LD approximation for the laser-ion interaction nor the auxiliary atomic level is needed in the present scheme.

quant-ph↗
arXiv2002

Quantum computation with two-level trapped cold ions beyond Lamb-Dicke limit

We propose a simple scheme for implementing quantum logic gates with a string of two-level trapped cold ions outside the Lamb-Dicke limit. Two internal states of each ion are used as one computational qubit (CQ) and the collective vibration of ions acts as the information bus, i.e., bus qubit (BQ). Using the quantum dynamics for the laser-ion interaction as described by a generalized Jaynes-Cummings model, we show that quantum entanglement between any one CQ and the BQ can be coherently manipulated by applying classical laser beams. As a result, universal quantum gates, i.e. the one-qubit rotation and two-qubit controlled gates, can be implemented exactly. The required experimental parameters for the implementation, including the Lamb-Dicke (LD) parameter and the durations of the applied laser pulses, are derived. Neither the LD approximation for the laser-ion interaction nor the auxiliary atomic level is needed in the present scheme.

quant-ph↗