M. Q. Weng

Spin dynamics in semiconductors

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

Kinetic theory of spin transport in n-type semiconductor quantum wells

We set up a set of many-body kinetic Bloch equations with spacial inhomogeneity. We re-examine the widely adopted quasi-independent electron model and show the inadequacy of this model in studying the spin transport. We further point out a new decoherence effect based on interference effect of electrons/spins with different momentum k along the direction of the diffusion, which is referred as “inhomogeneous broadening effect” in our paper. We show that this inhomogeneous broadening can cause spin decoherence alone even in the absence of the scattering and that the resulting decoherence can be more important than the dephasing effect due to the D’yakonov–Perel’ term together with the scattering. Our theory takes all the inhomogeneous broadening effect, the spin diffusion due to the spacial inhomogeneity and the spin dephasing into account and gets the results self-consistently. We further study the spin diffusion/transport of n-typed GaAs quantum wells in the steady state under different conditions, such a...

Spin-orbit coupling in bulk GaAs

Abstract We study the spin–orbit coupling in the whole Brillouin zone for GaAs using both the sp 3 s * d 5 and sp 3 s * 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.

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 periodically varies 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 double exponentially decays with two decay rates 1∕τ+ and 1∕τ−. In the high temperature regime, the average of these two rates quadratically varies with the grating wave vector q, i.e., (1∕τ++1∕τ−)∕2=Dsq2+1∕τs, with Ds and τs representing the spin diffusion coefficient and the average of the out-of-plane and the in-plane spin relaxation times, respectively. τ± calculated from our theory are in good agreement with the experimental data by Weber et al. [Phys. Rev. Lett. 98, 076604 (2007)]. By comparing Ds with and without the electron-electron Coulomb scattering, we calculate the contribution of Coulomb drag to the spin diffusion coefficient. With th...

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 scatterings 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. 93, 410 (2003)], can sustain the Coulomb scattering at high temperatures (∼200K). 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. 94, 236601 (2005)].

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

We study the Lande g-factor of conduction electrons in the L-valley of bulk GaAs and AlAs by using a three-band k⋅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⊥ and g∥. g⊥ is close to the free electron Lande 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.

Manipulation of spin dephasing in InAs quantum wires

Abstract The spin dephasing due to the Rashba spin–orbit coupling, especially its dependence on the direction of the electric field is studied in InAs quantum wire. We find that the spin dephasing is strongly affected by the angle of Rashba effective magnetic field and the applied magnetic field (AMF). The nonlinearity in spin dephasing time versus the direction of the electric field shows the potential to manipulate the spin lifetime in spintronic device. Moreover, we figure out a quantity that can well represent the inhomogeneous broadening of the system which may help us to understand the many-body spin dephasing due to the Rashba effect.