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

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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.