Moving Beyond a Simple Model of Luminescence Rings in Quantum Well   Structures
  The dramatic appearance of luminescence rings with radius of several hundred microns in quantum well structures can be understood through a fairly simple nonlinear model of the diffusion and recombination of electrons and holes in a driven nonequilibrium system. The ring corresponds to the boundary between a positive hole gas and a negative electron gas in steady state. While this basic effect is now well understood, we discuss several other experimental results which can not be explained by this simple model. 
D. Snoke, S. Denev, Y. Liu, S. Simon, R. Rapaport, G. Chen, L.   Pfeiffer, K. West