自旋光电流与电流诱导的电子自旋极化——半导体自旋电子学的新进展

文章通过自旋光电流与Kerr效应的实验，介绍在自旋一轨道耦合的体系中，如何通过自旋注入来驱动电子的运动产生电流，又如何反过来通过电子的运动或电流在系统中产生自旋激化．实验结果表明，自旋轨道耦合不但给我们提供了对自旋的操控手段，而且还给我们提供了用非磁性材料且无外加磁场条件下作为自旋源的新途径．     

自旋轨道耦合和自旋流的研究若干进展

近十年来,国内外科学工作者对自旋轨道耦合和自旋流作了很多深入的研究.文章介绍该领域的一些重要进展以及它的发展情况,包括介绍由自旋轨道耦合所引起的内在自旋霍尔效应和持续自旋流、自旋流的产生、自旋流的定义以及自旋流产生电场等.最后也讨论一些有待于解决的课题,以及对该领域的展望.     

自旋轨道耦合作用的等效磁场

自旋与电荷一样,是电子的固有属性,电子的周期性轨道运动产生的磁场与电子的自旋磁矩相互作用,这种磁相互作用就是自旋轨道相互作用。在原子物理学中,这种自旋轨道作用会影响原子光谱的精细结构,然而教材中缺少自旋轨道耦合作用在二维半导体材料中的微观描述。本文将引入Rashba和Dresselhaus自旋轨道耦合作用的哈密顿量,研究单电子在无外场下二维平面内运动,讨论一种或者两种自旋轨道耦合的哈密顿量表示。通过自旋与等效磁场耦合的塞曼能量表示,本文计算了本征态下不同自旋轨道耦合作用下的等效磁场,从而有助于探索二维半导体材料中不同自旋轨道耦合作用下的物理特性。     

旁耦合于介观环与铁磁电极量子点系统中的自旋极化输运

借助单杂质Anderson模型哈密顿量,及利用格林函数和运动方程等理论,研究旁耦合于介观环和铁磁电极的量子点系统中的极化输运特性.结果表明,通过调节点-环耦合强度、铁磁电极中的极化强度、磁矩相对取向及温度等,均能实现控制体系中自旋极化电流的目的,达到自旋阀效应.为此系统作为一种新的自旋电子材料提供理论依据.     

自旋-轨道耦合系统中的自旋流定义

通常的自旋流定义在描述自旋-轨道耦合系统中的自旋输运是不完整的与非物理的。文章作者提出在这类系统中自旋流的恰当定义。新定义的自旋流克服了通常定义下的本质缺点，可通过实验直接观测。     

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.     

δ势垒对多臂量子环中持续电流的影响

提出了含δ势垒的多臂量子环模型.研究发现总磁通为零时,持续电流随半导体环增大发生非周期性振荡,下臂因含δ势垒而获得最小的平均持续电流.AB磁通增强时,持续电流会发生周期性等幅振荡,并与电极的磁矩方向以及隧穿电子的自旋方向相关.两电极磁矩方向平行时,Rashba自旋轨道耦合具有改变持续电流相位和相位差的效应;两电极磁矩方向反平行时,Rashba自旋轨道耦合具有改变持续电流振幅的效应.各臂之间持续电流的不同与臂长和磁通分布的差异相关.在一定条件下,两种波函数所对应的持续电流是可分离的. 关键词： 多臂量子环 持续电流 δ势垒 Rashba自旋轨道耦合     

自旋轨道耦合量子点系统中的量子相干

研究了自旋轨道耦合量子点中的量子相干效应.运用输运电子的全计数统计方法计算系统的平均电流、散粒噪声和偏斜,发现体系存在自旋轨道耦合作用时,散粒噪声值随自旋轨道耦合常数的增加而减小.更重要的是,电流、噪声和偏斜随磁通周期性波动,并且波动周期不受自旋轨道耦合强度大小、自旋极化率以及动力学耦合不对称的影响.     

介观物理的粒子自旋轨道耦合和量子几何相位

本文较系统地但并非全面地讨论粒子的自旋轨道耦合。从基本概念到在介观物理的一些应用,特别着重讨论了自旋轨道耦合和量子几何相位的关系,也从作者的观点评论现代文献的一些有关问题和概述作者及其研究合作者的一些工作。本文讨论很大部分内容并不局限在介观尺度,而是相当普通的,可供物理各个领域研究参考。     

特殊结构的多臂量子环的持续电流

提出了中臂弯曲的多臂量子环模型,且是上臂最短和下臂最长的不等臂量子环.研究发现：总磁通为零时,持续电流随半导体环增大发生非周期性振荡,并与电极的磁矩方向及隧穿电子的自旋方向相关,下臂因为最长而获得最小的平均持续电流.AB磁通增强时,持续电流会发生周期性振荡,各臂之间明显出现相互制约的现象.各臂持续电流之间的差异与臂长和磁通分布相关,Rashba自旋轨道耦合具有改变持续电流相位和相位差的效应.在一定条件下,两种波函数所对应的持续电流是可分离的. 关键词： 多臂量子环 持续电流 Rashba自旋轨道耦合     

Persistent spin current in a mesoscopic hybrid ring with spin-orbit coupling

We investigate the equilibrium property of a mesoscopic ring with a spin-orbit interaction. It is well known that, for a normal mesoscopic ring threaded by a magnetic flux, the electron acquires a Berry phase that induces the persistent (charge) current. Similarly, the spin of an electron acquires a spin Berry phase traversing a ring with a spin-orbit interaction. It is this spin Berry phase that induces a persistent spin current. To demonstrate its existence, we calculate the persistent spin current without an accompanying charge current in the normal region in a hybrid mesoscopic ring. We point out that this persistent spin current describes the real spin motion and can be observed experimentally.     

Persistent current in a magnetized Rashba ring

We theoretically study the persistent currents flowing in a Rashba quantum ring subjected to the Rashba spin-orbit interaction. By introducing uniform or nonuniform magnetization into the ring, we find that a nonzero persistent charge current circulates in the ring, which stems from the original equilibrium spin current due to the Rashba spin-orbit interaction. Because of broken time reversal symmetry, the two oppositely flowing spin-up and spin-down charge currents of the equilibrium spin current are no longer equal, and so a net persistent charge current can flow in the system. It is also found that the persistent current can be modulated by the Fermi energy, the Rashba spin-orbit interaction strength and the magnetization in the ring. Moreover, the magnetization perpendicular to the ring plane can optimize the current. The persistent current flowing in the ring is a manifestation of the nonzero equilibrium spin current existing in the ring.     

Nonadiabatic Phase and Persistent Currents for System of Spin-1/2 Particles in Presence of Electromagnetic Fields and Spin-Orbit Interaction

We present a comprehensive view and details of calculations on Aharonov-Anandan phase for the charged particles in the external electric and magnetic fields for a nonadiabatic process. We derive, with consideration of a spin-orbit interaction and Zeemann Splitting, the persistent currents as a response to an Aharonov-Casher topological interference effect in one-dimensional mesoscopic ring. We also establish a connection to Berry adiabatic phase with deduced dynamical-nature dependence in the nonadiabatic process. The second quantization representation has also been employed in exhibition of persistent currents in the many-body case.     

Anomalous magnetic response of a quasi-periodic mesoscopic ring in presence of Rashba and Dresselhaus spin-orbit interactions

We investigate the properties of persistent charge current driven by magnetic flux in aquasi-periodic mesoscopic Fibonacci ring with Rashba and Dresselhaus spin-orbitinteractions. Within a tight-binding framework we work out individual state currentstogether with net current based on second-quantized approach. A significant enhancement ofcurrent is observed in presence of spin-orbit coupling and sometimes it becomes orders ofmagnitude higher compared to the spin-orbit interaction free Fibonacci ring. We alsoestablish a scaling relation of persistent current with ring size, associated with theFibonacci generation, from which one can directly estimate current for any arbitrary flux,even in presence of spin-orbit interaction, without doing numerical simulation. Thepresent analysis indeed gives a unique opportunity of determining persistent current andhas not been discussed so far.     

Aharonov-Bohm oscillations with spin: evidence for Berry's phase

We report a study of the Aharonov-Bohm effect, the oscillations of the resistance of a mesoscopic ring as a function of a perpendicular magnetic field, in a GaAs two-dimensional hole system with a strong spin-orbit interaction. The Fourier spectra of the oscillations reveal extra structure near the main peak whose frequency corresponds to the magnetic flux enclosed by the ring. A comparison of the experimental data with results of simulations demonstrates that the origin of the extra structure is the geometric (Berry) phase acquired by the carrier spin as it travels around the ring.     

Effects of geometry and linearly polarized cavity photons on charge and spin currents in a quantum ring with spin-orbit interactions

We calculate the persistent charge and spin polarization current inside a finite-width quantum ring of realistic geometry as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. The time evolution in the transient regime of the two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads is governed by a time-convolutionless non-Markovian generalized master equation. The electrons are correlated via Coulomb interaction. In addition, the ring is embedded in a photon cavity with a single mode of linearly polarized photon field, which is polarized either perpendicular or parallel to the charge transport direction. To analyze carefully the physical effects, we compare to the analytical results of the toy model of a one-dimensional (1D) ring of non-interacting electrons with spin-orbit coupling. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase ΔΦ = π, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin polarization currents of the 1D and 2D ring. Quantitatively, however, the spin polarization currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin polarization current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring, which is embedded in the photon cavity.     

Spin-polarized current in a Rashba ring pumped by a microwave field

We report a theoretical study on generation of a spin polarized charge current with arbitrary spin polarization, including the fully-spin-polarized current. In a two-terminal mesoscopic ring device, the Rashba spin-orbit coupling (RSOC) is considered as well as a microwave field applied on one of arms of the ring. It is shown that at zero external bias a spin current can be produced in addition to the usual charge current pumped by the microwave field, which is attributed to the the quantum interference effect of the RSOC induced spin precession phase. By varying the system parameters such as the microwave frequency and the RSOC strength, not only the magnitude but also the direction of the spin current can be efficiently controlled, moreover, the spin-polarization degree of the charge current can readily be tuned by these system parameters in the range [-1,1]. Since all the parameters can be controlled electrically in our study, the proposed device may shed light on the possibility of an all-electrical generation and tuning of a spin-polarized current in the field of the spintronics.     

Quantum interference of electrons in a ring: tuning of the geometrical phase

We calculate the oscillations of the dc conductance across a mesoscopic ring, simultaneously tuned by applied magnetic and electric fields orthogonal to the ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit coupling. They result from mixing of the dynamical phase, including the Zeeman spin splitting, and of geometric phases. By changing the applied fields, the geometric phase contribution to the conductance oscillations can be tuned from the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a realistic device, we also include nonzero backscattering at the connection between ring and contacts, and a random phase for electron wave function, accounting for dephasing effects.     

Characteristics of persistent spin current components in a quasi-periodic Fibonacci ring with spin–orbit interactions: Prediction of spin–orbit coupling and on-site energy

In the present work we investigate the behavior of all three components of persistent spin current in a quasi-periodic Fibonacci ring subjected to Rashba and Dresselhaus spin–orbit interactions. Analogous to persistent charge current in a conducting ring where electrons gain a Berry phase in presence of magnetic flux, spin Berry phase is associated during the motion of electrons in presence of a spin–orbit field which is responsible for the generation of spin current. The interplay between two spin–orbit fields along with quasi-periodic Fibonacci sequence on persistent spin current is described elaborately, and from our analysis, we can estimate the strength of any one of two spin–orbit couplings together with on-site energy, provided the other is known.     

Mesoscopic effects in adiabatic spin pumping

We show that temporal shape modulations (pumping) of a quantum dot in the presence of spin-orbital coupling lead to a finite dc spin current. Depending on the strength of the spin-orbit coupling, the spin current is polarized perpendicular to the plane of the two-dimensional electron gas, or has an arbitrary direction subject to mesoscopic fluctuations. We analyze the statistics of the spin and charge currents in the adiabatic limit for the full crossover from weak to strong spin-orbit coupling.