The effects of momentum scattering on exciton motion: observation of nondiffusive transport

Summary form only given. In the study of exciton motion in semiconductors and molecular crystals, it is usually assumed that the mean free path is sufficiently short compared with the relevant length scale that diffusive behavior, governed by Fick´s law, is expected. In this case, the exponential decay in time of an optically created grating (e.g., in four-wave mixing, FWM) has a quadratic dependence on the inverse grating spacing 1//spl Lambda/. Our work shows, however, that in high-quality bulk GaAs at low temperature, the momentum scattering rate is reduced, leading to a long mean free path, which is easily observed for length scales <10 /spl mu/m through a nonquadratic dependence of grating decay rate on 1//spl Lambda/. The nominally exponential decay and subquadratic dependence on 1//spl Lambda/ are accurately predicted by a solution of the quantum transport equation as a function of momentum scattering rate.