Spin diffusion of a spin-polarized two-dimensional electron system in semiconductor heterostructures

通过数值求解量子输运方程，研究了III-V型半导体异质结构中二维电子气体（2DEG）的自旋输运性质。结果表明，导致三维稀薄自旋极化^3He和氢的反常自旋扩散和自旋波的“自旋-旋转项”在低温下也存在于二维简并电子系统中。计算得到的纵向和横向自旋扩散系数表现出不同的温度依赖关系，这是由于与每个扩散过程相关的电子-电子散射中可用的相空间不同。在实际半导体的2DEG中，除了电子-电子（e-e）散射外，电子-杂质散射也会影响自旋扩散，而本理论并未包括电子-杂质散射。频繁的电子杂质散射会使我们无法观测到计算中预测的电子散射效应。从实验载流子迁移率出发，利用广义爱因斯坦关系估计了电子杂质散射对自旋扩散的贡献。估计表明，e-杂质和e-e散射在低温（< 10 K）下的高迁移率样品中具有相当的贡献，这表明我们需要非常高质量的样品来观察“自旋效应”。我们已经开始测量AlGaAs/GaAs/InGaAs量子阱中的自旋扩散和自旋弛豫。自旋极化电子在前AlGaAs层中产生光，然后它们漂移通过pn结，并在InGaAs中捕获并进行辐射衰变。初步的时间分辨偏振测量表明，电子在到达井底之前部分自旋松弛。结构或材料的优化是获得高自旋极化电子气体的必要条件。

We have investigated spin tranport properties of a two-dimensional electron gas (2DEG) in heterostructures of III-V semiconductors by numerically solving the quantum transport equation. It is shown that the 'spin-rotation term', which leads to anomalous spin diffusion and spin waves in three-dimensional rarefied spin polarized ^3He and Hydrogen, is also present in two-dimensional degenerate electron systems at low temperatures. Calculated longitudinal and transverse spin diffusion coefficients show different temperature dependence due to the fact that the phase space available in electron-electron scatterings associated with each diffusion process is different.In 2DEG of the actual semiconductors, in addition to electron-electron (e-e) scattering, electron-impurity scattering, which is not included in the present theory, also affects the spin diffusion.The frequent e-impurity scattering could prevent us from observing the effects of e-e scattering predicted in our calculations. We have estimated the contribution of e-impurity scattering to the spin diffusion from the experimental carrier mobilities by the generalized Einstein's relation. The estimation shows that both e-impurity and e-e scatterings have comparable contributions in the high mobility samples at low temperatures (< 10 K), indicating we need very high-quality samples to observe 'spin-rotation effects'.We have started measuring spin diffusion and spin relaxation in AlGaAs/GaAs/InGaAs quantum wells. The spin-polarized electrons are photogenerated in the front AlGaAs layer, then they drift through the pn junction, and captured in the InGaAs well to radiative decay. The preliminary time-resolved polarization measurements show that electrons are partially spin-relaxed before they reach the bottom of the well. The structural or material optimization is requisite to obtain a highly spin-polarized electron gas.