Coupled spin-charge drift-diffusion approach for a two-dimensional electron gas with Rashba spin-orbit coupling

Based on kinetic equations for the density matrix, drift-diffusion equations are derived for a two-dimensional electron gas with Rashba spin-orbit coupling. Universal results are obtained for the weak-coupling case. Most interesting is the observation that with increasing spin-orbit coupling strengths there is a sharp transition between spin diffusion and ballistic spin transport. For strong spin-orbit coupling, when the elastic scattering time is much larger than the spin relaxation time, undamped spin-coherent waves are identified. The existence of these long-lived spin-coherent states is confirmed by exact analytical results obtained from microscopic kinetic equations valid in the ballistic regime.