自旋轨道耦合系统中的自旋流与自旋霍尔效应

作为自旋电子学的重要研究内容,如何在固态系统中产生、操控以及探测自旋流引起了研究人员的广泛兴趣.基于自旋轨道耦合的自旋霍尔效应为在非磁性半导体中产生自旋流提供了一种有效途径.然而,在具有自旋轨道耦合的系统中,自旋流并不守恒.如何理解这点并恰当地表述相应的连续性方程,成为自旋输运研究的基本问题之一.本文主要综述自旋轨道耦合系统中自旋流与自旋霍尔效应方面的研究进展.引入SU(2)规范势后,自旋流满足协变形式的连续性方程,该方程保证了SU(2)Kubo公式在不同规范固定下的自洽性.利用SU(2)场强张量,可以直接得到自旋密度和自旋流在SU(2)外场中受到的白旋力,该力在只有U(1)磁场时对应于Stern-Gerlach力.由于依赖杂质散射的外在自旋霍尔效应很难被利用,内在自旋霍尔效应的概念被提出:在非磁半导体中,U(1)电场会诱导出自旋流并导致系统边缘处的自旋积累.自旋霍尔效应已经在半导体和金属材料中被观察到.虽然在干净的二维电子气中自旋霍尔电导率是一普适常数e/8π,但杂质对它的影响却引起了人们的高度关注.通过引入退相干效应,自旋霍尔效应中杂质效应的一些令人困惑的理论结果,则得到清晰的解释.此外,本文还将介绍具有层间隧穿的双层二维电子气中的自旋输运现象.在能量简并点附近,自旋霍尔电导率和隧穿白旋电导率均会出现共振现象.当两层间的杂质势强度存在差异时,隧穿自旋电导率随门压的变化曲线呈现出非对称性,显示出自旋二极管效应.

SPIN CURRENT AND SPIN HALL EFFECT IN SYSTEMS WITH SPIN-ORBIT COUPLING

Generating,manipulating and detecting a spin current are regarded as main tasks of spintronics.Spin Hall effect provides an alternative route to generate the spin current in semiconductors without spin injection.However,the spin current does not conserve in systems with spin-orbit coupling.Recently,much attention has been absorbed on the understanding of such an issue.We review the recent research progresses on the spin current and spin Hall effect in systems with spin-orbit coupling.In terms of SU(2)gauge potentials,the naturally defined spin current obeys a covariant continuity equation.This equation plays a key role in keeping the consistency of the SU(2)Kubo formula with different gauge fixings.After introducing the SU(2)field strength tensor,the spin force that electrons experience in the presence of the SU(2)gauge fields can be easily obtained.It reduces to the Stern-Gerlach force if only the U(1)magnetic field is present.Since there are seemly less application of the extrinsic spin Hall effect which depends on the impurity scattering,more attentions were paid to the intrinsic spin Hall effect recently.In this effect,aU(1)electric field can induce a transverse spin current and lateral spin accumulations in nonmagnetic semiconductors.The spin Hall effect has been observed in both semiconductors and metals.The spin Hall conductivity is a universal constant in the clean twodimensional electron gas,while the impurity effect on this conductivity used to be quite controversial which can be resolved if the dephasing effect is considered.In this review,the spin transport in the bilayer electron gas is also discussed.Both the spin Hall conductivity and tunnelling spin conductivity exhibit resonant peaks at the energy-degeneracy point.When taking into account the difference of strengthes of impurity potentials between layers,the tunnelling spin conductivity manifests an asymmetric feature with respect to the gate voltage,i.e.,a spin diode behavior is expected to occur in this bilayer system.