arXiv subjects

M. W. Wu

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

At least 127 records · Page 7Linked to original sources

arXiv2007

Anisotropic spin transport in GaAs quantum wells in the presence of competing Dresselhaus and Rashba spin-orbit-coupling strengths

Aiming at the optimization of the spin diffusion length in (001) GaAs quantum wells, we explore the effect of the anisotropy of the spin-orbit coupling on the competition between the Rashba and the Dresselhaus spin-orbit couplings by solving the kinetic spin Bloch equations with the electron-phonon and the electron-electron scattering explicitly included. For identical strengths of the Rashba and the Dresselhaus spin-orbit couplings, the spin diffusion length shows strong anisotropy not only for the spin polarization direction but also for the spin diffusion direction. Two special directions are used seeking for the large diffusion length: ($\bar{1}$10) and (110). Without the cubic term of the Dresselhaus spin-orbit coupling and with the identical Dresselhaus and Rashba strengths, infinite diffusion lengths can be obtained {\em either} for the spin diffusion/injection direction along $(\bar110)$, regardless of the direction of spin polarization, {\em or} for the spin polarization along $(110)$, regardless of the direction of the spin diffusion/injection. However, the cubic Dresselhaus term cannot be neglected, resulting into a finite spin diffusion length which decreases with the temperature and the electron density, and of which the anisotropy for the spin diffusion direction and spin polarization direction is maintained. For the spin diffusion/injection direction along ($\bar{1}$10), the spin diffusion length increases first with the increase of the Rashba strength (from 0) which can be tuned by the external gate voltage; when the Rashba strength is slightly smaller than (in stead of equal to) the Dresselhaus strength, the diffusion length reaches its maximum; and then it decreases with the Rashba strength.

cond-mat.mtrl-sci↗

arXiv2007

Effect of initial spin polarization on spin dephasing and electron g factor in a high-mobility two-dimensional electron system

We have investigated the spin dynamics of a high-mobility two-dimensional electron system (2DES) in a GaAs--Al$_{0.3}$Ga$_{0.7}$As single quantum well by time-resolved Faraday rotation (TRFR) in dependence on the initial degree of spin polarization, $P$, of the 2DES. From $P\sim 0$ to $P\sim 30$ %, we observe an increase of the spin dephasing time, $T_2^\ast$, by an order of magnitude, from about 20 ps to 200 ps, in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Furthermore, by applying an external magnetic field in the Voigt configuration, also the electron $g$ factor is found to decrease for increasing $P$. Fully microscopic calculations, by numerically solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the most salient features of the experiments, {\em i.e}., a dramatic decrease of spin dephasing and a moderate decrease of the electron $g$ factor with increasing $P$. We show that both results are determined dominantly by the Hartree-Fock contribution of the Coulomb interaction.

cond-mat.mtrl-sci↗

arXiv2007

Spin diffusion/transport in $n$-type GaAs quantum wells

The spin diffusion/transport in $n$-type (001) GaAs quantum well at high temperatures ($\ge120$ K) is studied by setting up and numerically solving the kinetic spin Bloch equations together with the Poisson equation self-consistently. All the scattering, especially the electron-electron Coulomb scattering, is explicitly included and solved in the theory. This enables us to study the system far away from the equilibrium, such as the hot-electron effect induced by the external electric field parallel to the quantum well. We find that the spin polarization/coherence oscillates along the transport direction even when there is no external magnetic field. We show that when the scattering is strong enough, electron spins with different momentums oscillate in the same phase which leads to equal transversal spin injection length and ensemble transversal injection length. It is also shown that the intrinsic scattering is already strong enough for such a phenomena. The oscillation period is almost independent on the external electric field which is in agreement with the latest experiment in bulk system at very low temperature [Europhys. Lett. {\bf 75}, 597 (2006)]. The spin relaxation/dephasing along the diffusion/transport can be well understood by the inhomogeneous broadening, which is caused by the momentum-dependent diffusion and the spin-orbit coupling, and the scattering. The scattering, temperature, quantum well width and external magnetic/electric field dependence of the spin diffusion is studied in detail.

cond-mat.mtrl-sci↗

arXiv2007

Spin relaxation time, spin dephasing time and ensemble spin dephasing time in $n$-type GaAs quantum wells

We investigate the spin relaxation and spin dephasing of $n$-type GaAs quantum wells. We obtain the spin relaxation time $T_1$, the spin dephasing time $T_2$ and the ensemble spin dephasing time $T_2^{\ast}$ by solving the full microscopic kinetic spin Bloch equations, and we show that, analogous to the common sense in an isotropic system for conduction electrons, $T_1$, $T_2$ and $T_2^{\ast}$ are identical due to the short correlation time. The inhomogeneous broadening induced by the D'yakonov-Perel term is suppressed by the scattering, especially the Coulomb scattering, in this system.

cond-mat.mtrl-sci↗

arXiv2006

Spin relaxation in an InAs quantum dot in the presence of terahertz driving fields

The spin relaxation in a 1D InAs quantum dot with the Rashba spin-orbit coupling under driving THz magnetic fields is investigated by developing the kinetic equation with the help of the Floquet-Markov theory, which is generalized to the system with the spin-orbit coupling, to include both the strong driving field and the electron-phonon scattering. The spin relaxation time can be effectively prolonged or shortened by the terahertz magnetic field depending on the frequency and strength of the terahertz magnetic field. The effect can be understood as the sideband-modulated spin-phonon scattering. This offers an additional way to manipulate the spin relaxation time.

cond-mat.mtrl-sci↗

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 relaxation in $n$-type GaAs quantum wells from a full microscopic approach

We perform a full microscopic investigation on the spin relaxation in $n$-type (001) GaAs quantum wells with Al$_{0.4}$Ga$_{0.6}$As barrier due to the D'yakonov-perel' mechanism from nearly 20 K to the room temperature by constructing and numerically solving the kinetic spin Bloch equations. We consider all the relevant scattering such as the electron--acoustic-phonon, the electron--longitudinal-optical-phonon, the electron--nonmagnetic-impurity and the electron-electron Coulomb scattering to the spin relaxation. The spin relaxation times calculated from our theory with a fitting spin splitting parameter are in good agreement with the experimental data by Ohno {\em et al.} [Physica E {\bf 6}, 817 (2000)] over the whole temperature regime (from 20 K to 300 K). The value of the fitted spin splitting parameter agrees with many experiments and theoretical calculations. We further show the temperature dependence of the spin relaxation time under various conditions such as electron density, impurity density and well width. We predict a peak solely due to the Coulomb scattering in the spin relaxation time at low temperature ($<50$ K) in samples with low electron density ({\em e.g.}, density less than $1 \times 10^{11}$ cm$^{-2}$) but high mobility. This peak disappears in samples with high electron density ({\em e.g.} $2 \times 10^{11}$ cm$^{-2}$) and/or low mobility. The hot-electron spin kinetics at low temperature is also addressed with many features quite different from the high temperature case predicted.

cond-mat.mtrl-sci↗

arXiv2006

Control of spin relaxation in semiconductor double quantum dots

We propose a scheme to manipulate the spin relaxation in vertically coupled semiconductor double quantum dots. Up to {\em twelve} orders of magnitude variation of the spin relaxation time can be achieved by a small gate voltage applied vertically on the double dot. Different effects such as the dot size, barrier height, inter-dot distance, and magnetic field on the spin relaxation are investigated in detail. The condition to achieve a large variation is discussed.

cond-mat.mes-hall↗

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↗

arXiv2006

Spin relaxation under identical Dresselhaus and Rashba coupling strengths in GaAs quantum wells

Spin relaxation under identical Dresselhaus and Rashba coupling strengths in GaAs quantum wells is studied in both the traditional collinear statistics, where the energy spectra do not contain the spin-orbit coupling terms, and the helix statistics, where the spin-orbit couplings are included in the energy spectra. We show that there is only marginal difference between the spin relaxation times obtained from these two different statistics. We further show that with the cubic term of the Dresselhaus spin-orbit coupling included, the spin relaxation time along the (1,1,0) direction becomes finite, although it is still much longer than that along the other two perpendicular directions. The properties of the spin relaxation along this special direction under varies conditions are studied in detail.

cond-mat.mtrl-sci↗

arXiv2006

Hole spin dephasing in $p$-type semiconductor quantum wells

Hole spin dephasing time due to the D'yakonov-Perel' mechanism in $p$-type GaAs (100) quantum wells with well separated light-hole and heavy-hole bands is studied by constructing and numerically solving the kinetic spin Bloch equations. We include all the spin-conserving scattering such as the hole-phonon and the hole-nonmagnetic impurity as well as the hole-hole Coulomb scattering in our calculation. Different effects such as the temperature, the hole density, the impurity density and the Rashba coefficient on the spin dephasing are investigated in detail. We also show that the Coulomb scattering makes marked contribution to the spin dephasing. The spin dephasing time can either increase or decrease with temperature, hole/impurity density or the inclusion of the Coulomb scattering depending on the relative importance of the spin-orbit coupling and the scattering. It is also shown that due to the different spin-orbit coupling strengthes, many spin dephasing properties of holes are quite different from those of electrons.

cond-mat.mtrl-sci↗

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↗

arXiv2005

Spin-dependent hole quantum transport in Aharonov-Bohm ring structure: possible schemes for spin filter

We study the Aharonov-Bohm (AB) effect in two-dimensional mesoscopic frame in hole systems. We show that differing from the AB effect in electron systems, due to the presence of both the heavy hole and the light hole, the conductances not only show the normal spin-unresolved AB oscillations, but also become spin-separated. Some schemes for spin filter based on the abundant interference characteristics are proposed.

cond-mat.mes-hall↗

arXiv2005

Schottky-barrier induced spin dephasing in spin injection

An ensemble Monte Carlo method is used to study the spin injection through a ferromagnet-semiconductor junction where a Schottky barrier is formed. It is shown that the Schottky-barrier-induced electric field which is confined in the depletion region and parallel to the injection direction, is very large. This electric field can induce an effective magnetic field due to the Rashba effect and cause strong spin dephasing.

cond-mat.mtrl-sci↗

arXiv2005

Spin-Hall effect in two-dimensional mesoscopic hole systems

The spin Hall effect in two dimensional hole systems is studied by using the four-terminal Landauer-Büttiker formula with the help of Green functions. The spin Hall effect exists even when there are {\em not} any correlations between the spin-up and -down heavy holes (light holes) and when the $Γ$-point degeneracy of the heavy hole and light hole bands is lifted by the confinement or recovered by the strain. When only a heavy hole charge current without any spin polarization is injected through one lead, under right choice of lead voltages, one can get a pure heavy (light) hole spin current, combined with a possible impure light (heavy) hole spin current from another two leads. The spin Hall coefficients of both heavy and light holes depend on the Fermi energy, devise size and the disorder strength. It is also shown that the spin Hall effect of two dimensional hole systems is much more robust than that of electron systems with the Rashba spin-orbit coupling and the spin Hall coefficients do not decrease with the system size but tend to some nonzero values when the disorder strength is smaller than some critical value.

cond-mat.mes-hall↗

arXiv2005

Control of spin coherence in $n$-type GaAs quantum wells using strain

We show that the bulk-inversion-asymmetry-type strain-induced spin-orbit coupling can be used to effectively modify the Dresselhaus spin splitting in (001) GaAs quantum wells with small well width and the resulting spin lifetime can be increased by two orders of magnitude to nanoseconds under right conditions. The efficiency of this strain manipulation of the spin dephasing time under different conditions such as temperature, electric field and electron density is investigated in detail.

cond-mat.mtrl-sci↗

arXiv2004

Formation and decay of electron-hole droplets in diamond

We study the formation and decay of electron-hole droplets in diamonds at both low and high temperatures under different excitations by master equations. The calculation reveals that at low temperature the kinetics of the system behaves as in direct-gap semiconductors, whereas at high temperature it shows metastability as in traditional indirect-gap semiconductors. Our results at low temperature are consistent with the experimental findings by Nagai {\em et al.} [Phys. Rev. B {\bf 68}, 081202 (R) (2003)]. The kinetics of the e-h system in diamonds at high temperature under both low and high excitations is also predicted.

cond-mat.mtrl-sci↗

arXiv2004

Intense terahertz laser fields on a two-dimensional electron gas with Rashba spin-orbit coupling

The spin-dependent density of states and the density of spin polarization of an InAs-based two-dimensional electron gas with the Rashba spin-orbit coupling under an intense terahertz laser field are investigated by utilizing the Floquet states to solve the time-dependent Schrödinger equation. It is found that both densities are strongly affected by the terahertz laser field. Especially a terahertz magnetic moment perpendicular to the external terahertz laser field in the electron gas is induced. This effect can be used to convert terahertz electric signals into terahertz magnetic ones efficiently.

cond-mat.mtrl-sci↗