Investigation On The Spin Dynamics Of Two-dimensional Electron Systems With Spin-orbit Couplings

Spin relaxation and spin dephasing are processes that lead to spin equilib-rium and thus of great importance for spintronics. Understanding spin relaxation of electrons can be helpful for the manipulation of electron spin in nanodevices. In this thesis, we firstly investigate spin relaxation of electrons in strained semi-conductor and analyze the effect of the strain on the spin-relaxation time. Then we study the effect of the electron-electron interaction on spin relaxation in two-dimensional electron gases. Finally, we investigate the strain dependence of the efficiency of spin manipulation in a quantum well. This thesis is organized as follows:1. In Chapter 1, we give a brief review of spintroncics. In Chapter 2, we briefly introduce the computing method and mechanisms of spin relaxation.2. In Chapter 3, the density-matrix formalism is applied to calculate the spin-relaxation time for two-dimensional systems with a hierarchy of spin-orbit couplings, such as Rashba-type, Dresselhaus-type and strain-induced. It is found that the spin-relaxation time can be infinite if those coupling strengthsα,β,γ1 andγ2 satisfy either conditionα=β,γ1= 0 orα=-β,γ2= 0, which correspond to the vanishing Yang-Mills fields. The effect caused by the application of an external magnetic field is also discussed. For this case, the spin-relaxation time can also be infinite if the spin-orbit coupling strengths and the direction of the magnetic field satisfy eitherα=β,γ1= 0,φ=π/2,θ= tan-1 (2α/γ2) orα= -β,γ2= 0,φ= 0,θ=-tan-1(2α/γ1), which are just the conditions of vanishing Yang-Mills fields. Finally, we propose a criterion:the spin-relaxation time can be infinite large when the Yang-Mills fields are zero.3. In Chapter 4, we study the spin dynamics of two dimensional electron gases with Rashba spin-orbit coupling by taking account of electron-electron in-teractions. The diffusion equations for charge and spin densities are derived by making use of the path-integral approach and the quasiclassical Green's func-tion. Analyzing the effect of the interactions, we show that the spin-relaxation time an be enhanced by the electron-electron interaction. The x component or y component of spin densities possesses an infinite spin-relaxation time whenτ′s/τxxe= 1 orτ′s/τyye= 1.4. In Chapter 5, we show that the efficiency of manipulating electron spin in semiconductor quantum wells can be enhanced by tuning the strain strength. The effect combining intrinsic and strain-induced spin-orbit couplings varies for differ-ent systems, which provides an alternative route to understand the experimental phenomena brought in by the strain. We analyze four types of systems com-bining intrinsic and strain-induced spin-orbit couplings, such as D+R, D+D, R+R and R+D type. The contribution to the electron-dipole-spin-resonance intensity induced by strain can be changed through adjusting the direction of the ac electric field in the x-y plane of the quantum well and tuning the strain strengths.