Oscillatory Dyakonov-Perel spin dynamics in two-dimensional electron gases

Abstract

Optical pump-probe measurements of spin dynamics at temperatures down to $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are described for a series of (001)-oriented $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ quantum well samples containing high mobility two-dimensional electron gases (2DEGs). For well widths ranging from $5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and 2DEG sheet densities from $1.75\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3.5\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, the evolution of a small injected spin population is found to be a damped oscillation rather than an exponential relaxation, consistent with the quasi-collision-free regime of the Dyakonov-Perel spin dynamics. A Monte Carlo simulation method is used to extract the spin-orbit-induced electron spin precession frequency $\ensuremath{\mid}\ensuremath{\Omega}({k}_{F})\ensuremath{\mid}$ and electron momentum scattering time $\ensuremath{\tau}_{p}{}^{*}$ at the Fermi wave vector. The spin decay time passes through a minimum at a temperature corresponding to the transition from collision-free to collision-dominated regimes and $\ensuremath{\tau}_{p}{}^{*}$ is found to be close to the ensemble momentum scattering time ${\ensuremath{\tau}}_{p}$ obtained from Hall measurements of electron mobility. The values of $\ensuremath{\mid}\ensuremath{\Omega}({k}_{F})\ensuremath{\mid}$ give the Dresselhaus or bulk inversion asymmetry (BIA) coefficient of spin-orbit interaction as a function of electron confinement energy in the quantum wells and show, qualitatively, the behavior expected from $k∙p$ theory.