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M. W. Wu

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

At least 91 records · Page 5Linked to original sources
arXiv2010

Singlet-triplet relaxation in multivalley silicon single quantum dots

We investigate the singlet-triplet relaxation due to the spin-orbit coupling together with the electron-phonon scattering in two-electron multivalley silicon single quantum dots, using the exact diagonalization method and the Fermi golden rule. The electron-electron Coulomb interaction, which is crucial in the electronic structure, is explicitly included. The multivalley effect induced by the interface scattering is also taken into account. We first study the configuration with a magnetic field in the Voigt configuration and identify the relaxation channel of the experimental data by Xiao {\em et al.} [Phys. Rev. Lett. {\bf 104}, 096801 (2010)]. Good agreement with the experiment is obtained. Moreover, we predict a peak in the magnetic-field dependence of the singlet-triplet relaxation rate induced by the anticrossing of the singlet and triplet states. We then work on the system with a magnetic field in the Faraday configuration, where the different values of the valley splitting are discussed. In the case of large valley splitting, we find the transition rates can be effectively manipulated by varying the external magnetic field and the dot size. The intriguing features of the singlet-triplet relaxation in the vicinity of the anticrossing point are analyzed. In the case of small valley splitting, we find that the transition rates are much smaller than those in the case of large valley splitting, resulting from the different configurations of the triplet states.

cond-mat.mes-hall↗
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↗
arXiv2010

Spin relaxation due to random Rashba spin-orbit coupling in GaAs (110) quantum wells

We investigate the spin relaxation due to the random Rashba spin-orbit coupling in symmetric GaAs (110) quantum wells from the fully microscopic kinetic spin Bloch equation approach. All relevant scatterings, such as the electron-impurity, electron--longitudinal-optical-phonon, electron--acoustic-phonon, as well as electron-electron Coulomb scatterings are explicitly included. It is shown that our calculation reproduces the experimental data by Müller {\em et al.} [Phys. Rev. Lett. {\bf 101}, 206601 (2008)] for a reasonable choice of parameter values. We also predict that the temperature dependence of spin relaxation time presents a peak in the case with low impurity density, which originates from the electron-electron Coulomb scattering.

cond-mat.mes-hall↗
arXiv2010

Effect of spin-conserving scattering on Gilbert damping in ferromagnetic semiconductors

The Gilbert damping in ferromagnetic semiconductors is theoretically investigated based on the $s$-$d$ model. In contrast to the situation in metals, all the spin-conserving scattering in ferromagnetic semiconductors supplies an additional spin relaxation channel due to the momentum dependent effective magnetic field of the spin-orbit coupling, thereby modifies the Gilbert damping. In the presence of a pure spin current, we predict a new contribution due to the interplay of the anisotropic spin-orbit coupling and a pure spin current.

cond-mat.mtrl-sci↗
arXiv2010

Effect of nonequilibrium phonons on hot-electron spin relaxation in n-type GaAs quantum wells

We have studied the effect of nonequilibrium longitudinal optical phonons on hot-electron spin relaxation in $n$-type GaAs quantum wells. The longitudinal optical phonons, due to the finite relaxation rate, are driven to nonequilibrium states by electrons under an in-plane electric field. The nonequilibrium phonons then in turn influence the electron spin relaxation properties via modifying the electron heating and drifting. The spin relaxation time is elongated due to the enhanced electron heating and thus the electron-phonon scattering in the presence of nonequilibrium phonons. The frequency of spin precession, which is roughly proportional to the electron drift velocity, can be either increased (at low electric field and/or high lattice temperature) or decreased (at high electric field and/or low lattice temperature). The nonequilibrium phonon effect is more pronounced when the electron density is high and the impurity density is low.

cond-mat.mtrl-sci↗
arXiv2009

Theory of the spin relaxation of conduction electrons in silicon

A realistic pseudopotential model is introduced to investigate the phonon-induced spin relaxation of conduction electrons in bulk silicon. We find a surprisingly subtle interference of the Elliott and Yafet processes affecting the spin relaxation over a wide temperature range, suppressing the significance of the intravalley spin-flip scattering, previously considered dominant, above roughly 120 K. The calculated spin relaxation times $T_1$ agree with the spin resonance and spin injection data, following a $T^{-3}$ temperature dependence. The valley anisotropy of $T_1$ and the spin relaxation rates for hot electrons are predicted.

cond-mat.mtrl-sci↗
arXiv2009

Electron spin relaxation in $p$-type GaAs quantum wells

We investigate electron spin relaxation in $p$-type GaAs quantum wells from a fully microscopic kinetic spin Bloch equation approach, with all the relevant scatterings, such as the electron-impurity, electron-phonon, electron-electron Coulomb, electron-hole Coulomb and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings explicitly included. From this approach, we examine the relative importance of the D'yakonov-Perel' and Bir-Aronov-Pikus mechanisms in wide ranges of temperature, hole density, excitation density and impurity density, and present a phase-diagram--like picture showing the parameter regime where the D'yakonov-Perel' or Bir-Aronov-Pikus mechanism is more important. By including more hole subbands and bands in our model, we are able to study spin dynamics at high hole density. It is shown that the Bir-Aronov-Pikus mechanism can surpass the D'yakonov-Perel' mechanism in some temperature regime with sufficiently high hole density for various impurity and excitation densities. We also discover that in the impurity-free case the temperature regime where the Bir-Aronov-Pikus mechanism is more efficient than the D'yakonov-Perel' one is around the hole Fermi temperature for high hole density, regardless of excitation density. However, in the high impurity density case with the impurity density being identical to the hole density, this regime is roughly from the electron Fermi temperature to the hole Fermi temperature. Particularly, the Bir-Aronov-Pikus mechanism can dominate the spin relaxation in the {\em whole} temperature regime of the investigation (from 5 K to 300 K) in the case with high impurity and very low excitation densities, since the electron (hole) Fermi temperature is... (The remaining is omitted due to the limit of the space)

cond-mat.mtrl-sci↗
arXiv2009

Comment on "Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature"

We comment on the conclusion by Ma et al. [Appl. Phys. Lett. {\bf 94}, 241112 (2009)] that the Elliott-Yafet mechanism is more important than the D'yakonov-Perel' mechanism at high carrier density in intrinsic bulk CdTe at room temperature. We point out that the spin relaxation is solely from the D'yakonov-Perel' mechanism. The observed peak in the density dependence of spin relaxation time is exactly what we predicted in a recent work [Phys. Rev. B {\bf 79}, 125206 (2009)].

cond-mat.mes-hall↗
arXiv2009

Hole spin relaxation in [001] strained asymmetric Si/SiGe and Ge/SiGe quantum wells

Hole spin relaxation in [001] strained asymmetric Si/Si$_{0.7}$Ge$_{0.3}$ (Ge/Si$_{0.3}$Ge$_{0.7}$) quantum wells is investigated in the situation with only the lowest hole subband being relevant. The effective Hamiltonian of the lowest hole subband is obtained by the subband Löwdin perturbation method in the framework of the six-band Luttinger ${\bf k}\cdot{\bf p}$ model, with sufficient basis functions included. The lowest hole subband in Si/SiGe quantum wells is light-hole like with the Rashba spin-orbit coupling term depending on momentum both linearly and cubically, while that in Ge/SiGe quantum wells is a heavy hole state with the Rashba spin-orbit coupling term depending on momentum only cubically. The hole spin relaxation is investigated by means of the fully microscopic kinetic spin Bloch equation approach, with all the relevant scatterings considered. It is found that the hole-phonon scattering is very weak, which makes the hole-hole Coulomb scattering become very important. The hole system in Si/SiGe quantum wells is generally in the strong scattering limit, while that in Ge/SiGe quantum wells can be in either the strong or the weak scattering limit. The Coulomb scattering leads to a peak in both the temperature and hole density dependences of spin relaxation time in Si/SiGe quantum wells, located around the crossover between the degenerate and nondegenerate regimes. Nevertheless, the Coulomb scattering leads to not only a peak but also a valley in the temperature dependence of spin relaxation time in Ge/SiGe quantum wells.... (The remaining is omitted due to the limit of space).

cond-mat.mtrl-sci↗
arXiv2009

A virtual intersubband spin-flip spin-orbit coupling induced spin relaxation in GaAs (110) quantum wells

A spin relaxation mechanism is proposed based on a second-order spin-flip intersubband spin-orbit coupling together with the spin-conserving scattering. The corresponding spin relaxation time is calculated via the Fermi golden rule. It is shown that this mechanism is important in symmetric GaAs (110) quantum wells with high impurity density. The dependences of the spin relaxation time on electron density, temperature and well width are studied with the underlying physics analyzed.

cond-mat.mes-hall↗
arXiv2009

Infinite spin diffusion length of any spin polarization along direction perpendicular to effective magnetic field from Dresselhaus and Rashba spin-orbit couplings with identical strengths in (001) GaAs quantum wells

In this note, we show that the latest spin grating measurement of spin helix by Koralek {\em et al.} [Nature {\bf 458}, 610 (2009)] provides strong evidence of the infinite spin diffusion length of any spin polarization along the direction perpendicular to the effective magnetic field from the Dresselhaus and Rashba spin-orbit couplings with identical strengths in $(001)$ GaAs quantum wells, predicted by Cheng {\em et al.} [Phys. Rev. B {\bf 75}, 205328 (2007)].

cond-mat.mtrl-sci↗
arXiv2009

Electron spin relaxation in n-type InAs quantum wires

We investigate the electron spin relaxation of $n$-type InAs quantum wires by numerically solving the fully microscopic kinetic spin Bloch equations with the relevant scattering explicitly included. We find that the quantum-wire size and the growth direction influence the spin relaxation time by modulating the spin-orbit coupling. Due to inter-subband scattering in connection with the spin-orbit interaction, spin-relaxation in quantum wires can show different characteristics from those in bulk or quantum wells and can be effectively manipulated by various means.

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

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied via the fully microscopic kinetic spin Bloch equation approach where all the scatterings, such as the electron-impurity, electron-phonon, electron-electron Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus mechanism) and the $s$-$d$ exchange scatterings, are explicitly included. The Elliot-Yafet mechanism is also incorporated. From this approach, we study the spin relaxation in both $n$-type and $p$-type Ga(Mn)As quantum wells. For $n$-type Ga(Mn)As quantum wells where most Mn ions take the interstitial positions, we find that the spin relaxation is always dominated by the DP mechanism in metallic region. Interestingly, the Mn concentration dependence of the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is because that the momentum scattering and the inhomogeneous broadening have different density dependences in the non-degenerate and degenerate regimes. For $p$-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of the spin relaxation time is also nonmonotonic and shows a peak. Differently, this behavior is because that the $s$-$d$ exchange scattering (or the Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn concentration regime at low (or high) temperature, whereas the DP mechanism determines the spin relaxation in the low Mn concentration regime. The Elliot-Yafet mechanism also contributes the spin relaxation at intermediate temperature. The spin relaxation time due to the DP mechanism increases with Mn concentration due to motional narrowing, whereas those due to the spin-flip mechanisms decrease with Mn concentration, which thus leads to the formation of the peak.... (The remaining is omitted due to the space limit)

cond-mat.mtrl-sci↗
arXiv2009

Electron spin relaxation in bulk III-V semiconductors from a fully microscopic kinetic spin Bloch equation approach

Electron spin relaxation in bulk III-V semiconductors is investigated from a fully microscopic kinetic spin Bloch equation approach where all relevant scatterings, such as, the electron--nonmagnetic-impurity, electron-phonon, electron-electron, electron-hole, and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings are explicitly included. The Elliot-Yafet mechanism is also fully incorporated. This approach offers a way toward thorough understanding of electron spin relaxation both near and far away from the equilibrium in the metallic regime. The dependence of the spin relaxation time on electron density, temperature, initial spin polarization, photo-excitation density, and hole density are studied thoroughly with the underlying physics analyzed. In contrast to the previous investigations in the literature, we find that: (i) In $n$-type materials, the Elliot-Yafet mechanism is {\em less} important than the D'yakonov-Perel' mechanism, even for the narrow band-gap semiconductors such as InSb and InAs. (ii) The density dependence of the spin relaxation time is nonmonotonic and we predict a {\em peak} in the metallic regime in both $n$-type and intrinsic materials. (iii) In intrinsic materials, the Bir-Aronov-Pikus mechanism is found to be negligible compared with the D'yakonov-Perel' mechanism. We also predict a peak in the temperature dependence of spin relaxation time which is due to the nonmonotonic temperature dependence of the electron-electron Coulomb scattering in intrinsic materials with small initial spin polarization. (iv) In $p$-type III-V semiconductors, ...... (the remaining is omitted here due to the limit of space)

cond-mat.mtrl-sci↗
arXiv2009

Spin diffusion in Si/SiGe quantum wells: spin relaxation in the absence of D'yakonov-Perel' relaxation mechanism

In this work, the spin relaxation accompanying the spin diffusion in symmetric Si/SiGe quantum wells without the D'yakonov-Perel' spin-relaxation mechanism is calculated from a fully microscopic approach. The spin relaxation is caused by the inhomogeneous broadening from the momentum-dependent spin precessions in spatial domain under a magnetic field in the Voigt configuration. In fact, this inhomogeneous broadening together with the scattering lead to an irreversible spin relaxation along the spin diffusion. The effects of scattering, magnetic field and electron density on spin diffusion are investigated. Unlike the case of spin diffusion in the system with the D'yakonov-Perel' spin-orbit coupling such as GaAs quantum wells where the scattering can either enhance or reduce spin diffusion depending on whether the system is in strong or weak scattering limit, the scattering in the present system has no counter-effect on the inhomogeneous broadening and suppresses the spin diffusion monotonically. The increase of magnetic field reduces the spin diffusion, while the increase of electron density enhances the spin diffusion when the electrons are degenerate but has marginal effect when the electrons are nondegenerate.

cond-mat.mtrl-sci↗
arXiv2008

Manganese-diffusion-induced n-doping in semiconductor structures containing Ga(Mn)As layers

Semiconductor structures using ferromagnetic semiconductors as spin injectors are promising systems for future spintronic devices. Here, we present combined photoluminescence (PL) and time-resolved magneto-optical experiments of a nominally nonmagnetic quantum well(QW) separated by a thin barrier from a ferromagnetic Ga(Mn)As layer. Due to the partial quenching of the PL, we conclude that there is a significant Mn backdiffusion into the QW. Moreover, from the time-resolved measurements, we infer that the Mn leads to n-type doping within the QW, and, in addition, strongly increases the electron spin dephasing time.

cond-mat.other↗