Intersubband spin-orbit coupling and spin splitting in symmetric quantum wells

In semiconductors with inversion asymmetry, spin-orbit coupling gives rise to the well-known Dresselhaus and Rashba effects. If one considers quantum wells with two or more conduction subbands, an additional, intersubband-induced spin-orbit term appears whose strength is comparable to the Rashba coupling, and which remains finite for symmetric structures. We show that the conduction band spin splitting due to this intersubband spin-orbit coupling term is negligible for typical III-V quantum wells.     

Electrical manipulation and measurement of spin properties of quantum spin Hall edge states

We study the effects of a gate-controlled Rashba spin-orbit coupling to quantum spin Hall edge states in HgTe quantum wells. A uniform Rashba coupling can be employed in tuning the spin orientation of the edge states while preserving the time-reversal symmetry. We introduce a sample geometry where the Rashba coupling can be used in probing helicity by purely electrical means without requiring spin detection, application of magnetic materials or magnetic fields. In the considered setup a tilt of the spin orientation with respect to the normal of the sample leads to a reduction in the two-terminal conductance with current-voltage characteristics and temperature dependence typical of Luttinger liquid constrictions.     

HgTe/HgCdTe量子阱中巨大电子Rashba自旋分裂

主要研究具有倒置能带结构的n-HgTe/HgCdTe第三类量子阱Shubnikov-de Haas（SdH）振荡中的拍频现象.发现在量子阱中电子存在强烈的Rashba自旋分裂，通过对SdH振荡进行三种不同方法的分析：SdH振荡对1/B关系的快速傅里叶变换、SdH振荡中拍频节点分析和对SdH振荡拍频数值拟合，得到了完全一致的电子Rashba自旋分裂能量（28—36 meV）. 关键词： n-HgTe/HgCdTe Shubnikov-de Haas振荡 Rashba自旋分裂     

Zero field spin splitting in asymmetric quantum wells

Spin splitting of asymmetric quantum wells is theoretically investigated in the absence of any electric field, including the contribution of interface-related Rashba spin-orbit interaction as well as linear and cubic Dresselhaus spin-orbit interaction. The effect of interface asymmetry on three types of spin-orbit interaction is discussed. The results show that interface-related Rashba and linear Dresselhaus spin-orbit interaction can be increased and cubic Dresselhaus spin-orbit interaction can be decreased by well structure design. For wide quantum wells, the cubic Dresselhaus spin-orbit interaction dominates under certain conditions, resulting in decreased spin relaxation time.     

Spin-orbit interaction in coupled quantum wells

We theoretically investigate the spin-orbit interaction in GaAs/AlxGa1 x As coupled quantum wells. We consider the contribution of the interface-related Rashba term as well as the linear and cubic Dresselhaus terms to the spin splitting. For the coupled quantum wells which bear an inherent structure inversion asymmetry, the same probability density distribution of electrons in the two step quantum wells results in a large spin splitting from the interface term. If the widths of the two step quantum wells are different, the electron probability density in the wider step quantum well is considerably higher than that in the narrower one, resulting in the decrease of the spin splitting from the interface term. The results also show that the spin splitting of the coupled quantum well is not significantly larger than that of a step quantum well.     

Spin-orbit berry phase in a quantum loop

We have found a manifestation of spin-orbit Berry phase in the conductance of a mesoscopic loop with Rashba spin-orbit coupling placed in an external magnetic field perpendicular to the loop plane. In detail, the transmission probabilities for a straight quantum wire and for a quantum loop made of the same wire have been calculated and compared with each other. The difference between them has been investigated and identified with a manifestation of spin-orbit Berry phase. The non-adiabaticity effects at small radii of the loop have been found as well.     

Thermodynamic behaviour of Rashba quantum dot in the presence of magnetic field

The thermodynamic properties of an In Sb quantum dot have been investigated in the presence of Rashba spin–orbit interaction and a static magnetic field. The energy spectrum and wave-functions for the system are obtained by solving the Schrodinger wave-equation analytically. These energy levels are employed to calculate the specific heat, entropy,magnetization and susceptibility of the quantum dot system using canonical formalism. It is observed that the system is susceptible to maximum heat absorption at a particular value of magnetic field which depends on the Rashba coupling parameter as well as the temperature. The variation of specific heat shows a Schottky-like anomaly in the low temperature limit and rapidly converges to the value of 2kB with the further increase in temperature. The entropy of the quantum dot is found to be inversely proportional to the magnetic field but has a direct variation with temperature. The substantial effect of Rashba spin–orbit interaction on the magnetic properties of quantum dot is observed at low values of magnetic field and temperature.     

半导体中自旋轨道耦合及自旋霍尔效应

本文主要评述和介绍半导体微结构中自旋轨道耦合的研究和最近的研究进展。我们细致地讨论了半导体微结构中自旋轨道耦合的物理起源和窄带隙半导体量子阱中的自旋霍尔效应。我们发现目前国际上广泛采用的线性Rashba模型在较大的电子平面波矢处失效:即自旋轨道耦合导致的能带自旋劈裂不再随电子波矢的增加而增加,而是开始下降,即出现强烈的非线性行为。这种非线性的行为起源于导带和价带间耦合的减弱。这种非线性行为还会导致电子的D’yakonov-Perel’自旋弛豫速率在较高能量处下降,与线性模型的结果完全相反。在此基础上,我们构造统一描述电子和空穴自旋霍尔效应的理论框架。我们的方法可以非微扰地计入自旋轨道耦合对本征自旋霍尔效应的影响。我们将此方法应用于强自旋轨道耦合的情形,即窄带隙CdHgTe/CdTe半导体量子阱。我们发现调节外电场或量子阱的阱宽可以作为导致量子相变和本征自旋霍尔效应的开关。我们的工作可能会为区别和实验验证本征自旋霍尔效应提供物理基础。     

低温InGaAs/InAlAs多量子阱的分子束外延生长与表征

采用气源分子束外延(GSMBE)生长了低温InGaAs材料,研究了生长温度及As压对InGaAs材料性质的影响,得到优化的生长条件为:生长温度为300 ℃、As压为77.3 kPa。通过Be掺杂,并采用In0.52Al0.48As/In0.53Ga0.47As多量子阱结构,将材料的方块电阻提高到1.632106 /Sq,载流子数密度降低至1.0581014 cm-3。X射线衍射结果表明:InGaAs多量子阱材料具有较高的晶体质量。这种Be掺杂InGaAs多量子阱材料缺陷密度大且电阻率高,是制作太赫兹光电导天线较理想的基质材料。收稿日期:; 修订日期:     

Rashba-effect-induced localization in quantum networks

We study a quantum network extending in one dimension (chain of square loops connected at one vertex) made up of quantum wires with Rashba spin-orbit coupling. We show that the Rashba effect may give rise to an electron localization phenomenon similar to the one induced by magnetic field. This localization effect can be attributed to the spin precession due to the Rashba effect. We present results both for the spectral properties of the infinite chain and for linear transport through a finite-size chain connected to leads. Furthermore, we study the effect of disorder on the transport properties of this network.     

Transport polygonal properties in a multi-terminal regular quantum ring with Rashba spin-orbit coupling

<正>Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling(SOC) are investigated analytically using quantum networks and the transport matrix method.The results show that conductances remain at exactly the same values when the output leads are located at axisymmetric positions.However,for the nonaxisymmetrical case,there is a phase difference between the upper and lower arm,which leads to zero conductances appearing periodically.An isotropy of the conductance is destroyed by the Rashba SOC effect in the axisymmetric case. In addition,the position of zero conductance is regulated with the strength of the Rashba SOC.     

Spin Splitting in Different Semiconductor Quantum Wells

We theoretically investigate the spin splitting in four undoped asymmetric quantum wells in the absence of external electric field and magnetic field. The quantum well geometry dependence of spin splitting is studied with the Rashba and the Dresselhaus spin-orbit coupling included. The results show that the structure of quantum well plays an important role in spin splitting. The Rashba and the Dresselhaus spin splitting in four asymmetric quantum wells are quite different. The origin of the distinction is discussed in this work.     

Quantum spin Hall and quantum valley Hall effects in trilayer graphene and their topological structures

The present study pertains to the trilayer graphene in the presence of spin orbit coupling to probe the quantum spin/valley Hall effect. The spin Chern-number C_s for energy-bands of trilayer graphene having the essence of intrinsic spin–orbit coupling is analytically calculated. We find that for each valley and spin, C_s is three times larger in trilayer graphene as compared to single layer graphene. Since the spin Chern-number corresponds to the number of edge states,consequently the trilayer graphene has edge states, three times more in comparison to single layer graphene. We also study the trilayer graphene in the presence of both electric-field and intrinsic spin–orbit coupling and investigate that the trilayer graphene goes through a phase transition from a quantum spin Hall state to a quantum valley Hall state when the strength of the electric field exceeds the intrinsic spin coupling strength. The robustness of the associated topological bulk-state of the trilayer graphene is evaluated by adding various perturbations such as Rashba spin–orbit(RSO) interaction αR, and exchange-magnetization M. In addition, we consider a theoretical model, where only one of the outer layers in trilayer graphene has the essence of intrinsic spin–orbit coupling, while the other two layers have zero intrinsic spin–orbit coupling.Although the first Chern number is non-zero for individual valleys of trilayer graphene in this model, however, we find that the system cannot be regarded as a topological insulator because the system as a whole is not gaped.     

Rashba自旋-轨道相互作用影响下量子盘中强耦合磁极化子性质的研究

本文基于Lee-Low-Pines幺正变换法,采用Tokuda改进的线性组合算符法研究了Rashba自旋-轨道相互作用效应下量子盘中强耦合磁极化子的性质.结果表明,磁极化子的相互作用能Eint的取值随量子盘横向受限强度ω0、外磁场的回旋频率ωc、电子-LO声子耦合强度α和量子盘厚度L的变化均与磁极化子的状态性质密切相关;磁极化子的平均声子数N随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在Rashba自旋-轨道相互作用效应影响下磁极化子的有效质量将劈裂为m*+,m*-两种,它们随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在研究量子盘中磁极化子问题时,电子-LO声子耦合和Rashba自旋-轨道相互作用效应的影响不可忽略,但Rashba自旋-轨道相互作用和极化子效应对磁极化子的影响只有在电子运动的速率较慢时显著.     

AlGaN/GaN量子阱中子带的Rashba自旋劈裂和子带间自旋轨道耦合作用研究

首先把本征值方程投影到导带的子空间中, 进而得到AlGaN/GaN量子阱中第一、二子带的Rashba自旋劈裂系数(α ₁, α ₂)和子带间自旋-轨道耦合系数η₁₂. 然后自恰求解薛定谔方程和泊松方程计算了不同栅压的量子阱中的α ₁, α ₂和η₁₂, 并分别讨论了量子阱阱层、左右异质结界面和垒层对它们的贡献. 结果表明可以通过栅压来调节自旋-轨道耦合系数, 子带间自旋轨道耦合系数η₁₂比Rashba自旋劈裂系数α ₁, α ₂小, 但基本在同一数量级.     

Spin precession and electron spin polarization wave in [001]-grown quantum wells

We theoretically study the spatial behaviors of spin precessions modulated by an effective magnetic field in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field, we find some laws of spin precession in the system, by which we explain some previous phenomena of spin precession, and predict a controllable electron spin polarization wave in [001]-grown quantum wells. The shape of the wave, like water wave, mostly are ellipse-like or circle-like, and the wavelength is anisotropic in the quantum wells with two unequal coupling strengths of the Rashba and Dresselhaus interactions, and is isotropic in the quantum wells with only one spin orbit interaction.     

Second harmonic generation in a disk shaped quantum dot in the presence of spin-orbit interaction

The second harmonic generation (SHG) coefficients for a disk shaped quantum dot (DSQD) in the magnetic field are studied in the presence of spin-orbit interactions (SOI). The spin-orbit terms we have used in our calculations are both Rashba and Dresselhaus. We have shown that the presence of SOI modifies the SHG terms. In addition, it has been shown that SOI coupling terms influence the spectrum of DSQD resulting in defined changes in the harmonic generation.     

Spin-orbit gauge and quantum spin Hall effect

We have shown that the non-Abelian spin-orbit gauge field strength of the Rashba and Dresselhaus interactions, when split into two Abelian field strengths, the Hamiltonian of the system can be re-expressed as a Landau level problem with a particular relation between the two coupling parameters. The quantum levels are created with up and down spins with opposite chirality and leads to the quantum spin Hall effect.     

Role of orbital dynamics in spin relaxation and weak antilocalization in quantum dots

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.     

Dependence of Spin-orbit Parameters in AlxGa1-xN/GaN Quantum Wells on the Al Composition of the Barrier

In this paper, we obtain considerable spin-orbit (SO) parameters in Al_xGa_1-xN/GaN quantum wells (QWs) with sheet carrier concentration N_s =120×10 ¹¹/cm². With increasing Al content (x) of the barrier, the SO parameters increase as a whole, and the two major contributions are found to be the decrease of the expansion region of the envelope functions and the increase of the polarized electric field in the well. Compared with the Rashba parameters for the first two subbands, the intersubband SO parameter is a bit smaller and varies more slowly with x. The results indicate the SO parameters, especially the Rashba parameters can be engineered by the Al composition of the barrier, which may be helpful to the spin manipulation of III-nitride low-dimensional heterostructures.