【作者】 王春明；

【导师】 雷啸霖；

【作者基本信息】 上海交通大学 ， 理论物理， 2008， 博士

【摘要】 在这篇论文里,我们探讨了二维自旋轨道耦合电子系统的自旋霍尔效应以及其他自旋相关的输运性质。自旋霍尔效应的讨论主要是考虑线性Rashba自旋轨道耦合导致的内禀自旋霍耳效应。在第一章我们回顾了最近自旋轨道耦合系统中关于自旋霍尔效应和体自旋极化的理论和实验方面的研究进展。第二章我们首先从维象和Dirac方程两个角度讨论了原子中自旋轨道耦合的来源,接着概括了固体中自旋轨道耦合现象,特别针对线性Rashba耦合,我们讨论了这个系统的本征值和其他一些特征量。第三章探讨了自旋流的定义以及存在的一些问题,采用通常的定义,计算并得出了没有杂质时的自旋霍尔电导,并且从微扰和一般性论证两个角度讨论了杂质导致的自旋霍尔流消失的效应。第四章我们研究了高频THz光场作用下的二维Rashba系统,对于高频系统,无杂质的假设是物理的。我们发现这时的自旋霍尔流将以THz光场的频率为频率振荡,并且它的振荡幅度也会受到光场强度的影响。观察到了自旋流振幅和光强的非线性依赖关系,自旋霍尔流的幅度可以达到一个最大值,然后下降。另外,耦合系数对于自旋流的振荡幅度也有很强的影响。这些特征为利用光学方法调控自旋电子学器件提供了一种可能性。由于振荡的自旋流与体的自旋极化的时间导数直接联系,这样通过测量体自旋极化可以获得传统定义下的理论自旋流密度预言,这必将为研究最重要的如何联系实验测量与理论预言带来巨大的意义。考虑到直流情况下,杂质散射导致自旋霍尔电导完全消失的结论是针对抛物电子系统而言的,在第五章我们研究了非抛物能谱的情形。具体计算中考虑了Kane色散关系,我们发现包含顶角修正后杂质散射导致的自旋霍尔电导不再与无杂质的自旋霍尔电导抵消,这时的总自旋霍尔电导不仅依赖于非抛物的程度而且与散射势的平滑度有直接的关系,同时自旋霍尔电导与电子浓度成单调递增的关系。我们还讨论了自旋流在加上弱直流电场后的瞬态响应,观察到了与电流类似的“过冲”现象。第六章中利用平衡方程的方法研究了这种电子系统中其他一些输运量的性质。处理中我们同时考虑了杂质和声子散射,这时横向电荷电导仍旧等于零,而纵向电荷电导变得依赖于自旋轨道耦合强度。除了自旋极化的y分量S_y之外,其它的体自旋极化分量S_x、S_z等于零。y分量的自旋极化S_y在耦合比较强时可能会出现非线性的依赖于耦合的现象。声子散射在高温时对电荷迁移率的峰值结构和自旋极化有很强的抑制作用。在强场情况下,我们发现耦合能够增强电子冷却效应并且使得自旋极化出现平台结构,同时自旋极化的最大值是受材料耦合的限制的。这些特征与实验结果定性一致。我们还讨论了非抛物效应,发现对于这个相对较低密度和较强耦合的二维系统,非抛物性是非常重要的、不可忽略的。最后一章我们给出了结论和展望。更多还原

【Abstract】 In this dissertation we have made a detail investigation on spin Hall effect and related transport property in two-dimensional spin-orbital interaction electron system. Here the discussed spin Hall effect is in the regime of the intrinsic one which is induced by the spin-orbital interaction relied on the structure of the system rather than on the impurity scattering.In chapter one we review the recent theoretical and experimental research on the spin Hall effect and bulk spin polarization in spin-orbital coupling systems. We discuss the origin of the spin-orbital interaction in atom with both the phenomenological analysis and the rigorous Dirac equation, and describe the spin-orbital coupling in solid, especially, the Rashba coupling in chapter two. And finally we give the eigen energies and eigen states for Rashba system. In chapter three the definition of the spin current and some related problem are discussed. By using the conventional definition, we first calculate the spin-Hall conductivity without impurity, and then consider the system in the diffusive regime.We discuss the properties of the linear Rashba system in the presence of the THz optical field in chapter four. For high frequency situation the clean assumption is valid, we discuss the system without impurity. We find that the spin-Hall current oscillates with the frequency of the THz field and its amplitude is influenced by the optical intensity. We observe the nonlinearity between the spin-Hall current amplitude and the optical intensity, amplitude may reach a maximum and then decrease. These feature may provide a different way to control the spin polarization and the spintronic devices optically in nonmagnetic semiconductors. Since the oscillated spin current is directly related to the bulk spin polarization, this provides a direct method to study the relationship between the traditional spin current density and the experimental measurement. Since the precise cancellation of spin-Hall conductivity in the Rashba model of two dimensional electron systems occurs only for the parabolic energy spectrum, in chapter five we consider the nonparabolic situation. For nonparabolic Kane dispersion, we find that the cancellation effect from the spin-dependent part of the ladder current vertex in the Born approximation for impurity scattering will not occur. The total spin Hall conductivity is finite and its value not only relates to the nonparabolic degree of the electron energy dispersion but also strongly depends on the smoothness of the disorder, and it increases with the increasing electron density. The overshoot phenomena analogous with the one of charge current is observed for the transient situation.In chapter six, we discuss other properties of this Rashba system by the balance equation approach. In the case of realistic impurity and phonon scatterings, the longitudinal conductivity is shown to depend on the spin-orbital coupling strength, though the transverse conductivity remains zero. The nonlinear effect of spin-orbital coupling on the y component of the spin polarization is observed, while the other components still vanish. At high temperatures, the phonon scattering has an impact on the suppression of the mobility peak and the spin polarization. In the nonlinear case, we find that the spin-orbital coupling may enhance the electron cooling, a clear saturation of spin polarization appears at high electric field, and the maximum y component of spin polarization the system can reach is limited by the spin-split strength. These features are in qualitative agreement with experimental results. We have also studied the effect of nonparabolicity due to Rashba coupling and find that it plays an appreciable role for low density and strong spin-orbital coupling system.The last chapter gives the conclusion and outlook of this dissertation.更多还原