Bose-Einstein condensation of excitons in cuprous oxide

Free excitons provide the only experimental system other than helium in which the behavior of particles with mass is known to follow Bose-Einstein statistics. I present experimental observations of the kinetic energy distribution of excitons in the direct-gap semiconductor Cu$\sb2$O, both the triplet orthoexciton state and the singlet paraexciton state. The density and temperature of the exciton gas closely follow the phase boundary for Bose-Einstein condensation. At the highest densities, the lower-lying paraexcitons take on an anomalous energy distribution with a sharp, high-energy edge. This odd distribution of particle energies may be associated with Bose-Einstein condensation into a state with nonzero momentum. Indeed, the excitons leave the region of their creation at supersonic velocities.In addition to the exerimental observations, I also present theoretical models for several aspects of this non-equilibrium system. I model the equilibration of a nearly-ideal boson gas and find that a significant time is required for the approach to condensation. I also propose a new mechanism for spin-flip conversion between the orthoexciton and paraexciton states. Finally, I model the temperature and density of the excitons in steady state based on known classical kinetic effects in semiconductors, and I estimate the effects of Bose-Einstein statistics on these processes.U of I OnlyETDs are only available to UIUC Users without author permissio