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J. L. Cheng

Publications and source records attributed to J. L. Cheng.

32 records · Page 2Linked to original sources
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

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

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 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

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↗
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↗
arXiv2004

Hole spin relaxation in semiconductor quantum dots

Hole spin relaxation time due to the hole-acoustic phonon scattering in GaAs quantum dots confined in quantum wells along (001) and (111) directions is studied after the exact diagonalization of Luttinger Hamiltonian. Different effects such as strain, magnetic field, quantum dot diameter, quantum well width and the temperature on the spin relaxation time are investigated thoroughly. Many features which are quite different from the electron spin relaxation in quantum dots and quantum wells are presented with the underlying physics elaborated.

cond-mat.mtrl-sci↗
arXiv2003

Spin relaxations in semiconductor quantum dots

The spin relaxation time due to the electron-acoustic phonon scattering in GaAs quantum dots is studied after the exact diagonalization of the electron Hamiltonian with the spin-orbit coupling. Different effects such as the magnetic field, the quantum dot size, the temperature as well as the electric field on the spin relaxation time are investigated in detail. Moreover, we show that the perturbation method widely used in the literature is inadequate in accounting for the electron structure and therefore the spin relaxation time.

cond-mat↗
arXiv2003

Manipulation of spin dephasing in InAs quantum wires

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. The nonlinearity in spin dephasing time versus the direction of the electric field shows a potential evenue 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.

cond-mat↗