arXiv subjects

M. W. Wu

Publications and source records attributed to M. W. Wu.

At least 109 records · Page 6Linked to original sources

arXiv2008

A scheme for spin transistor with extremely large on/off current ratio

Quantum wires with periodic local Rashba spin-orbit couplings are proposed for a higher performance of spin field-effect transistor. Fano-Rashba quantum interference due to the spin-dependent modulated structure gives rise to a broad energy range of vanishingly small transmission. Tuning Rashba spin-orbit couplings can provide the on- or off-currents with extremely large on/off current ratios even in the presence of a strong disorder.

cond-mat.mes-hall↗

arXiv2008

Hole spin relaxation in $p$-type GaAs quantum wires

We investigate the spin relaxation of $p$-type GaAs quantum wires by numerically solving the fully microscopic kinetic spin Bloch equations. We find that the quantum-wire size influences the spin relaxation time effectively by modulating the energy spectrum and the heavy-hole--light-hole mixing of wire states. The effects of quantum-wire size, temperature, hole density, and initial polarization are investigated in detail. We show that, depending on the situation, the spin relaxation time can either increase or decrease with hole density. Due to the different subband structure and effects arising from spin-orbit coupling, many spin-relaxation properties are quite different from those of holes in the bulk or in quantum wells, and the inter-subband scattering makes a marked contribution to the spin relaxation.

cond-mat.mtrl-sci↗

arXiv2008

Spin orbit coupling in bulk ZnO and GaN

Using group theory and Kane-like $\mathbf{k\cdot p}$ model together with the Löwdining partition method, we derive the expressions of spin-orbit coupling of electrons and holes, including the linear-$k$ Rashba term due to the intrinsic structure inversion asymmetry and the cubic-$k$ Dresselhaus term due to the bulk inversion asymmetry in wurtzite semiconductors. The coefficients of the electron and hole Dresselhaus terms of ZnO and GaN in wurtzite structure and GaN in zinc-blende structure are calculated using the nearest-neighbor $sp^3$ and $sp^3s^\ast$ tight-binding models separately.

cond-mat.mtrl-sci↗

arXiv2008

Density dependence of spin relaxation in GaAs quantum well at room temperature

Carrier density dependence of electron spin relaxation in an intrinsic GaAs quantum well is investigated at room temperature using time-resolved circularly polarized pump-probe spectroscopy. It is revealed that the spin relaxation time first increases with density in the relatively low density regime where the linear D'yakonov-Perel' spin-orbit coupling terms are dominant, and then tends to decrease when the density is large and the cubic D'yakonov-Perel' spin-orbit coupling terms become important. These features are in good agreement with theoritical predictions by Lü {\em et al.} [Phys. Rev. B {\bf 73}, 125314 (2006)]. A fully microscopic calculation based on numerically solving the kinetic spin Bloch equations with both the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms included, reproduces the density dependence of spin relaxation very well.

cond-mat.mtrl-sci↗

arXiv2008

Kinetics of spin coherence of electrons in $n$-type InAs quantum wells under intense terahertz laser fields

Spin kinetics in $n$-type InAs quantum wells under intense terahertz laser fields is investigated by developing fully microscopic kinetic spin Bloch equations via the Floquet-Markov theory and the nonequilibrium Green's function approach, with all the relevant scattering, such as the electron-impurity, electron-phonon, and electron-electron Coulomb scattering explicitly included. We find that a {\em finite} steady-state terahertz spin polarization induced by the terahertz laser field, first predicted by Cheng and Wu [Appl. Phys. Lett. {\bf 86}, 032107 (2005)] in the absence of dissipation, exists even in the presence of all the scattering. We further discuss the effects of the terahertz laser fields on the spin relaxation and the steady-state spin polarization. It is found that the terahertz laser fields can {\em strongly} affect the spin relaxation via hot-electron effect and the terahertz-field-induced effective magnetic field in the presence of spin-orbit coupling. The two effects compete with each other, giving rise to {\em non-monotonic} dependence of the spin relaxation time as well as the amplitude of the steady state spin polarization on the terahertz field strength and frequency. The terahertz field dependences of these quantities are investigated for various impurity densities, lattice temperatures, and strengths of the spin-orbit coupling. Finally, the importance of the electron-electron Coulomb scattering on spin kinetics is also addressed.

cond-mat.mtrl-sci↗

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

Multi-valley spin relaxation in the presence of high in-plane electric fields in $n$-type GaAs quantum wells

Multi-valley spin relaxation in $n$-type GaAs quantum wells with in-plane electric field is investigated at high temperature by means of kinetic spin Bloch equation approach. The spin relaxation time first increases and then decreases with electric field, especially when the temperature is relatively low. We show that $L$ valleys play the role of a ``drain'' of the total spin polarization due to the large spin-orbit coupling in $L$ valleys and the strong $Γ$-$L$ inter-valley scattering, and thus can enhance spin relaxation of the total system effectively when the in-plane electric field is high. Under electric field, spin precession resulting from the electric-field-induced magnetic field is observed. Meanwhile, due to the strong $Γ$-$L$ inter-valley scattering as well as the strong inhomogeneous broadening in $L$ valleys, electron spins in $L$ valleys possess almost the same damping rate and precession frequency as those in $Γ$ valley. This feature still holds when a finite static magnetic field is applied in Voigt configuration, despite that the $g$-factor of $L$ valleys is much larger than that of $Γ$ valley. Moreover, it is shown that the property of spin precession of the whole system is dominated by electrons in $Γ$ valley. Temperature, magnetic field, and impurity can affect spin relaxation in low electric field regime. However, they are shown to have marginal influence in high electric field regime.

cond-mat.mtrl-sci↗

arXiv2008

Robust strongly-modulated transmission of a $T$-shaped structure with local Rashba interaction

We propose a scheme of spin transistor using a $T$-shaped structure with local Rashba interaction. A wide antiresonance energy gap appears due to the interplay of two types of interference, the Fano-Rashba interference and the structure interference. A large current from the gap area can be obtained via changing the Rashba strength and/or the length of the sidearm by using gate voltage. The robustness of the antiresonance gap against strong disorder is demonstrated and shows the feasibility of this structure for the real application.

cond-mat.mes-hall↗

arXiv2008

Control of spin coherence in semiconductor double quantum dots

We propose a scheme to manipulate the spin coherence in vertically coupled GaAs double quantum dots. Up to {\em ten} orders of magnitude variation of the spin relaxation and {\em two} orders of magnitude variation of the spin dephasing can be achieved by a small gate voltage applied vertically on the double dot. Specially, large variation of spin relaxation still exists at 0 K. In the calculation, the equation-of-motion approach is applied to obtain the electron decoherence time and all the relevant spin decoherence mechanisms, such as the spin-orbit coupling together with the electron--bulk-phonon scattering, the direct spin-phonon coupling due to the phonon-induced strain, the hyperfine interaction and the second-order process of electron-phonon scattering combined with the hyperfine interaction, are included. The condition to obtain the large variations of spin coherence is also addressed.

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↗

arXiv2008

Kinetic investigation on extrinsic spin Hall effect induced by skew scattering

The kinetics of the extrinsic spin Hall conductivity induced by the skew scattering is performed from the fully microscopic kinetic spin Bloch equation approach in $(001)$ GaAs symmetric quantum well. In the steady state, the extrinsic spin Hall current/conductivity vanishes for the linear-$\mathbf k$ dependent spin-orbit coupling and is very small for the cubic-$\mathbf k$ dependent spin-orbit coupling. The spin precession induced by the Dresselhaus/Rashba spin-orbit coupling plays a very important role in the vanishment of the extrinsic spin Hall conductivity in the steady state. An in-plane spin polarization is induced by the skew scattering, with the help of the spin-orbit coupling. This spin polarization is very different from the current-induced spin polarization.

cond-mat.mes-hall↗

arXiv2007

Spin relaxation due to the Bir-Aronov-Pikus mechanism in intrinsic and $p$-type GaAs quantum wells from a fully microscopic approach

We study the electron spin relaxation in intrinsic and $p$-type (001) GaAs quantum wells by constructing and numerically solving the kinetic spin Bloch equations. All the relevant scatterings are explicitly included, especially the spin-flip electron-heavy hole exchange scattering which leads to the Bir-Aronov-Pikus spin relaxation. We show that, due to the neglection of the nonlinear terms in the electron-heavy hole exchange scattering in the Fermi-golden-rule approach, the spin relaxation due to the Bir-Aronov-Pikus mechanism is greatly exaggerated at moderately high electron density and low temperature in the literature. We compare the spin relaxation time due to the Bir-Aronov-Pikus mechanism with that due to the D'yakonov-Perel' mechanism which is also calculated from the kinetic spin Bloch equations with all the scatterings, especially the spin-conserving electron-electron and electron-heavy hole scatterings, included. We find that, in intrinsic quantum wells, the effect from the Bir-Aronov-Pikus mechanism is much smaller than that from the D'yakonov-Perel' mechanism at low temperature, and it is smaller by no more than one order of magnitude at high temperature. In $p$-type quantum wells, the spin relaxation due to the Bir-Aronov-Pikus mechanism is also much smaller than the one due to the D'yakonov-Perel' mechanism at low temperature and becomes comparable to each other at higher temperature when the hole density and the width of the quantum well are large enough. We claim that unlike in the bulk samples, the Bir-Aronov-Pikus mechanism hardly dominates the spin relaxation in two-dimensional samples.

cond-mat.mtrl-sci↗

arXiv2007

Reexamination of spin decoherence in semiconductor quantum dots from equation-of-motion approach

The longitudinal and transversal spin decoherence times, $T_1$ and $T_2$, in semiconductor quantum dots are investigated from equation-of-motion approach for different magnetic fields, quantum dot sizes, and temperatures. Various mechanisms, such as the hyperfine interaction with the surrounding nuclei, the Dresselhaus spin-orbit coupling together with the electron--bulk-phonon interaction, the $g$-factor fluctuations, the direct spin-phonon coupling due to the phonon-induced strain, and the coaction of the electron--bulk/surface-phonon interaction together with the hyperfine interaction are included. The relative contributions from these spin decoherence mechanisms are compared in detail. In our calculation, the spin-orbit coupling is included in each mechanism and is shown to have marked effect in most cases. The equation-of-motion approach is applied in studying both the spin relaxation time $T_1$ and the spin dephasing time $T_2$, either in Markovian or in non-Markovian limit. When many levels are involved at finite temperature, we demonstrate how to obtain the spin relaxation time from the Fermi Golden rule in the limit of weak spin-orbit coupling. However, at high temperature and/or for large spin-orbit coupling, one has to use the equation-of-motion approach when many levels are involved. Moreover, spin dephasing can be much more efficient than spin relaxation at high temperature, though the two only differs by a factor of two at low temperature.

cond-mat.mtrl-sci↗

arXiv2007

Triplet-singlet relaxation in semiconductor single and double quantum dots

We study the triplet-singlet relaxation in two-electron semiconductor quantum dots. Both single dots and vertically coupled double dots are discussed. In our work, the electron-electron Coulomb interaction, which plays an important role in the electronic structure, is included. The spin mixing is caused by spin-orbit coupling which is the key to the triplet-singlet relaxation. We show that the selection rule widely used in the literature is incorrect unless near the crossing/anticrossing point in single quantum dots. The triplet/singlet relaxation in double quantum dots can be markedly changed by varying barrier height, inter-dot distance, external magnetic field and dot size.

cond-mat.mtrl-sci↗

arXiv2007

Non-Markovian hole spin kinetics in $p$-type GaAs quantum wells

Based on fully microscopic kinetic spin Bloch equation approach, we show the non-Markovian effect in spin dephasing of heavy holes in $p$-type GaAs quantum wells. The non-Markovian effect manifests itself in spin dephasing when the mean spin precession time is shorter than the momentum relaxation time. The spin dephasing becomes slower when the non-Markovian effect is important. Moreover, quantum spin beats due to the memory effect of the non-Markovian hole-longitudinal optical phonon scattering are predicted.

cond-mat.mtrl-sci↗

arXiv2007

Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system

We have studied the spin dynamics of a high-mobility two-dimensional electron system in a GaAs/Al_{0.3}Ga_{0.7}As single quantum well by time-resolved Faraday rotation and time-resolved Kerr rotation in dependence on the initial degree of spin polarization, P, of the electrons. By increasing the initial spin polarization from the low-P regime to a significant P of several percent, we find that the spin dephasing time, $T_2^\ast$, increases from about 20 ps to 200 ps; Moreover, $T_2^\ast$ increases with temperature at small spin polarization but decreases with temperature at large spin polarization. All these features are in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Measurements as a function of spin polarization at fixed electron density are performed to further confirm the theory. A fully microscopic calculation is performed by setting up and numerically solving the kinetic spin Bloch equations, including the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, with {\em all} the scattering explicitly included. We reproduce all principal features of the experiments, i.e., a dramatic decrease of spin dephasing with increasing $P$ and the temperature dependences at different spin polarizations.

cond-mat.mtrl-sci↗

arXiv2007

Detection of large magneto-anisotropy of electron spin dephasing in a high-mobility two-dimensional electron system in a $[001]$ GaAs/AlGaAs quantum well

In time-resolved Faraday rotation experiments we have detected an inplane anisotropy of the electron spin-dephasing time (SDT) in an $n$--modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As single quantum well. The SDT was measured with magnetic fields of $B\le 1$ T, applied in the $[110]$ and $[1\bar{1}0]$ inplane crystal directions of the GaAs quantum well. For fields along $[1\bar{1}0]$, we have found an up to a factor of about 2 larger SDT than in the perpendicular direction. Fully microscopic calculations, by numerically solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the experimental findings quantitatively. This quantitative analysis of the data allowed us to determine the relative strengths of Rashba and Dresselhaus terms in our sample. Moreover, we could estimate the SDT for spins aligned in the $[110]$ {\em inplane} direction to be on the order of several nanoseconds, which is up to two orders of magnitude larger than that in the perpendicular {\em inplane} direction.

cond-mat.mtrl-sci↗