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M. Q. Weng

Publications and source records attributed to M. Q. Weng.

At least 19 recordsLinked to original sources
arXiv2014

High-field charge transport on the surface of Bi$_2$Se$_3$

We present a theoretical study on the high-field charge transport on the surface of Bi$_2$Se$_3$ and reproduce all the main features of the recent experimental results, i.e., the incomplete current saturation and the finite residual conductance in the high applied field regime [Costache {\it et al.}, Phys. Rev. Lett. {\bf 112}, 086601 (2014)]. Due to the hot-electron effect, the conductance decreases and the current shows the tendency of the saturation with the increase of the applied electric field. Moreover, the electric field can excite carriers within the surface bands through interband precession and leads to a higher conductance. As a joint effect of the hot-electron transport and the carrier excitation, the conductance approaches a finite residual value in the high-field regime and the current saturation becomes incomplete. We thus demonstrate that, contrary to the conjecture in the literature, the observed transport phenomena can be understood qualitatively in the framework of surface transport alone. Furthermore, if a constant bulk conductance which is insensitive to the field is introduced, one can obtain a good quantitative agreement between the theoretical results and the experimental data.

cond-mat.mes-hall↗
arXiv2013

Ambipolar spin transport in (111) GaAs quantum wells

We present a microscopic theory for transport of the spin polarized charge density wave with both electrons and holes in the $(111)$ GaAs quantum wells. We analytically show that, contradicting to the commonly accepted belief, the spin and charge motions are bound together only in the fully polarized system but can be separated in the case of low spin polarization or short spin lifetime even when the spatial profiles of spin density wave and charge density wave overlap with each other. We further show that, the Coulomb drag between electrons and holes can markedly enhance the hole spin diffusion if the hole spin motion can be separated from the charge motion. In the high spin polarized system, the Coulomb drag can boost the hole spin diffusion coefficient by more than one order of magnitude.

cond-mat.mes-hall↗
arXiv2013

Quasi-bound states and Fano effect in T-shaped graphene nanoribbons

We study the quasi-bound state and the transport properties in the T-shaped graphene nanoribbon consisting of a metallic armchair-edge ribbon connecting to a zigzag-edge sidearm. We systematically study the condition under which there are quasi-bound states in the system for a wide range of the system size. It is found that when the width of the sidearm is about half of the width of the armchair leads, there is a quasi-bound state trapped at the intersection of the T-shape structure. The quasi-bound states are truly localized in the sidearm but have small continuum components in the armchair leads. The quasi-bound states have strong effect on the transport between the armchair leads through the Fano effect, but do not affect the transport between the armchair lead and the sidearm.

cond-mat.mes-hall↗
arXiv2012

Microscopic theory for the Doppler velocimetry of spin propagation in semiconductor quantum wells

We provide a microscopic theory for the Doppler velocimetry of spin propagation in the presence of spatial inhomogeneity, driving electric field and the spin orbit coupling in semiconductor quantum wells in a wide range of temperature regime based on the kinetic spin Bloch equation. It is analytically shown that under an applied electric field, the spin density wave gains a time-dependent phase shift $ϕ(t)$. Without the spin-orbit coupling, the phase shift increases linearly with time and is equivalent to a normal Doppler shift in optical measurements. Due to the joint effect of spin-orbit coupling and the applied electric field, the phase shift behaviors differently at the early and the later stages. At the early stage, the phase shifts are the same with or without the spin-orbit coupling. While at the later stage, the phase shift deviates from the normal Doppler one when the spin-orbit coupling is present. The crossover time from the early normal Doppler behavior to the anomalous one at the later stage is inversely proportional to the spin diffusion coefficient, wave vector of the spin density wave and the spin-orbit coupling strength. In the high temperature regime, the crossover time becomes large as a result of the decreased spin diffusion coefficient. The analytic results capture all the quantitative features of the experimental results, while the full numerical calculations agree quantitatively well with the experimental data obtained from the Doppler velocimetry of spin propagation [Yang {\it et al.}, Nat. Phys. {\bf 8}, 153 (2012)]. We further predict that the coherent spin precession, originally thought to be broken down at high temperature, is robust up to the room temperature for narrow quantum wells. We point out that one has to carry out the experiments longer to see the effect of the coherent spin precession at higher temperature due to the larger crossover time.

cond-mat.mes-hall↗
arXiv2010

Entanglement renormalization of anisotropic XY model

The renormalization group flows of the one-dimensional anisotropic XY model and quantum Ising model under a transverse field are obtained by different multiscale entanglement renormalization ansatz schemes. It is shown that the optimized disentangler removes the short-range entanglement by rotating the system in the parameter space spanned by the anisotropy and the magnetic field. It is understood from the study that the disentangler reduces the entanglement by mapping the system to another one in the same universality class but with smaller short range entanglement. The phase boundary and corresponding critical exponents are calculated using different schemes with different block sizes, look-ahead steps and truncation dimensions. It is shown that larger truncation dimension leads to more accurate results and that using larger block size or look-ahead step improve the overall calculation consistency.

cond-mat.str-el↗
arXiv2010

Spin dynamics in semiconductors

This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.

cond-mat.mtrl-sci↗
arXiv2009

Electron spin relaxation in cubic GaN quantum dots

The spin relaxation time $T_{1}$ in zinc blende GaN quantum dot is investigated for different magnetic field, well width and quantum dot diameter. The spin relaxation caused by the two most important spin relaxation mechanisms in zinc blende semiconductor quantum dots, {i.e.} the electron-phonon scattering in conjunction with the Dresselhaus spin-orbit coupling and the second-order process of the hyperfine interaction combined with the electron-phonon scattering, are systematically studied. The relative importance of the two mechanisms are compared in detail under different conditions. It is found that due to the small spin orbit coupling in GaN, the spin relaxation caused by the second-order process of the hyperfine interaction combined with the electron-phonon scattering plays much more important role than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.

cond-mat.mtrl-sci↗
arXiv2009

Decoherence and relaxation in the interacting quantum dot system

In this paper we study the low temperature kinetics of the electrons in the system composed of a quantum dot connected to two leads by solving the equation of motion. The decoherence and the relaxation of the system caused by the gate voltage noise and electron-phonon scattering are investigated. In order to take account of the strong correlation of the electrons in this system, the quasi-exact wave functions are calculated using an improved matrix product states algorithm. This algorithm enables us to calculate the wave functions of the ground state and the low lying excited states with satisfied accuracy and thus enables us to study the kinetics of the system more effectively. It is found that although both of these two mechanisms are proportional to the electron number operator in the dot, the kinetics are quite different. The noise induced decoherence is much more effective than the energy relaxation, while the energy relaxation and decoherence time are of the same order for the electron-phonon scattering. Moreover, the noise induced decoherence increases with the lowering of the dot level, but the relaxation and decoherence due to the electron-phonon scattering decrease.

cond-mat.str-el↗
arXiv2008

Supersolid Order of Frustrated Hard-Core Bosons in a Triangular Lattice System

We numerically demonstrate that a supersolid phase exists in a frustrated hard-core boson system on a triangular lattice over a wide range of interaction strength. In the infinite repulsion (Ising) limit, we establish a mapping to the same problem with unfrustrated hopping, which connects the supersolid to the known results in that case. The weak superfluidity can be destroyed or strongly enhanced by a next nearest neighbor hopping term, which provides valuable information for experimental realization of a supersolid phase on optical lattice.

cond-mat.str-el↗
arXiv2008

$L$-valley electron $g$ factor in bulk GaAs and AlAs

We study the Landé $g$-factor of conduction electrons in the $L$-valley of bulk GaAs and AlAs by using a three-band $\mathbf{k}\cdot\mathbf{p}$ model together with the tight-binding model. We find that the $L$-valley $g$-factor is highly anisotropic, and can be characterized by two components, $g_{\perp}$ and $g_{\|}$. $g_{\perp}$ is close to the free electron Landé factor but $g_{\|}$ is strongly affected by the remote bands. The contribution from remote bands on $g_{\|}$ depends on how the remote bands are treated. However, when the magnetic field is in the Voigt configuration, which is widely used in the experiments, different models give almost identical $g$-factor.

cond-mat.mtrl-sci↗
arXiv2008

Spin-orbit coupling in bulk GaAs

We study the spin-orbit coupling in the whole Brillouin zone for GaAs using both the $sp^3s^{\ast}d^5$ and $sp^3s^{\ast}$ nearest-neighbor tight-binding models. In the $Γ$-valley, the spin splitting obtained is in good agreement with experimental data. We then further explicitly present the coefficients of the spin splitting in GaAs $L$ and $X$ valleys. These results are important to the realization of spintronic device and the investigation of spin dynamics far away from equilibrium.

cond-mat.mtrl-sci↗
arXiv2008

Spin relaxation in $n$-type GaAs quantum wells with transient spin grating

By solving the kinetic spin Bloch equations, we study the time evolution of the transient spin grating, whose spin polarization varies periodically in real space, confined in (001) GaAs quantum wells. With this study we can investigate the properties of both the spin transport and the spin relaxation at the same time. The Fourier component of the spin signal decays double exponentially with two decay rates $1/τ_+$ and $1/τ_-$. In high temperature regime, the average of these two rates varies with the grating wave-vector $q$ quadratically, i.e., $(1/τ_++1/τ_-)/2=D_sq^2+1/\tildeτ_s$, with $D_s$ and $\tildeτ_s$ representing the spin diffusion coefficient and the average of the out-of-plane and the in-plane spin relaxation times respectively. $τ_{\pm}$ calculated from our theory are in good agreement with the experimental data by Weber {\em et al.} [Phys. Rev. Lett. {\bf 98}, 076604 (2007)]. By comparing $D_s$ with and without the electron-electron Coulomb scattering, we calculate the contribution of Coulomb drag to the spin diffusion coefficient. With the transient spin grating result, we further reveal the relations among different characteristic parameters such as spin diffusion coefficient $D_s$, spin relaxation time $τ_s$, and spin injection length $L_s$. We show that in the presence of the Dresselhaus and/or Rashba spin-orbit coupling, the widely used relation $L_s=\sqrt{D_sτ_s}$ is generally inaccurate and can even be very wrong in some special cases. We present an accurate way to extract the steady-state transport characteristic parameters from the transient spin grating signals.

cond-mat.mtrl-sci↗
arXiv2007

Remote-control spin filtering through a $T$-type structure

We propose a spin filter scheme using a $T$-stub waveguide. By applying a moderate magnetic field at the tip of the sidearm, this device can produce both large electric and spin current. The direction, polarization of the output spin current can be further adjusted electronically by a remote gate which tunes the length of the sidearm. The device is robust against the disorder.

cond-mat.mes-hall↗
arXiv2006

Spin dynamics in semiconductor nanostructures

We review our theoretical investigation on the spin relaxation/dephasing in spin precession and spin diffusion/transport in semiconductor nanostructures based on the kinetic spin Bloch equation approach.

cond-mat.mtrl-sci↗
arXiv2006

Spin Liquid Phase in Anisotropic Triangular Lattice Heisenberg Model: Exact diagonalization and density-matrix renormalization group calculations

Based on exact diagonalization and density matrix renormalization group (DMRG) method, we show that an anisotropic triangle lattice Heisenberg spin model has three distinct quantum phases. In particular, a spin-liquid phase is present in the weak interchain coupling regime, which is characterized by an exponentially-decaying spin correlator along the weaker coupling direction and a quasi-one-dimensional spin structure factor along the chain direction, in agreement with experiments on the Cs2CuCl4 compounds. In the obtained phase diagram, the spin liquid phase is found to persist up to a relatively large critical anisotropic coupling ratio J'/J=0.78, which is stabilized by strong quantum fluctuations, with a parity symmetry distinct from two magnetic ordered states in the stronger coupling regime.

cond-mat.str-el↗
arXiv2006

Intense terahertz laser fields on a quantum dot with Rashba spin-orbit coupling

We investigate the effects of the intense terahertz laser field and the spin-orbit coupling on single electron spin in a quantum dot. The laser field and the spin-orbit coupling can strongly affect the electron density of states and can excite a magnetic moment. The direction of the magnetic moment depends on the symmetries of the system, and its amplitude can be tuned by the strength and frequency of the laser field as well as the spin-orbit coupling.

cond-mat.mes-hall↗
arXiv2005

Diffusion and transport of spin pulses in an $n$-type semiconductor quantum well

We perform a theoretical investigation on the time evolution of spin pulses in an $n$-type GaAs (001) quantum well with and without external electric field at high temperatures by constructing and numerically solving the kinetic spin Bloch equations and the Poisson equation, with the electron-phonon, electron-impurity and electron-electron Coulomb scattering explicitly included. The effect of the Coulomb scattering, especially the effect of the Coulomb drag on the spin diffusion/transport is investigated and it is shown that the spin oscillations and spin polarization reverse along the direction of spin diffusion in the absence of the applied magnetic field, which were originally predicted in the absence of the Coulomb scattering by Weng and Wu [J. Appl. Phys. {\bf 93}, 410 (2003)], can sustain the Coulomb scattering at high temperatures ($\sim 200$ K). The results obtained are consistent with a recent experiment in bulk GaAs but at a very low temperature (4 K) by Crooker and Smith [Phys. Rev. Lett. {\bf 94}, 236601 (2005)].

cond-mat.mtrl-sci↗