Origin of the inner ring in photoluminescence patterns of quantum well   excitons
  In order to explain and model the inner ring in photoluminescence (PL) patterns of indirect excitons in GaAs/AlGaAs quantum wells (QWs), we develop a microscopic approach formulated in terms of coupled nonlinear equations for the diffusion, thermalization and optical decay of the particles. The origin of the inner ring is unambiguously identified: it is due to cooling of indirect excitons in their propagation from the excitation spot. We infer that in our high-quality structures the in-plane diffusion coefficient is about 10-30cm^2/s and the amplitude of the disorder potential is about 0.45meV. 
A. L. Ivanov, L. E. Smallwood, A. T. Hammack, Sen Yang, L. V. Butov   and A. C. Gossard