Auger decay, spin exchange, and their connection to Bose-Einstein condensation of excitons in Cu2O

In view of the recent experiments of O'Hara er al. [Phys. Rev. B 60, 10 565 (1999) and Phys. Rev. B 60, 10 872 (1999)] on excitons in Cu2O, we examine the interconversion between the angular-momentum triplet-state excitons and the angular-momentum singlet-state excitons by a spin-exchange process that has been overlooked in the past. We estimate the rate of this particle-conserving mechanism and find a substantially higher value than the rate of the Anger process considered so far. Based on this idea, we give a possible explanation of the recent experimental observations, and make certain predictions, with the most important bring that the singlet-state excitons in Cu2O is a very serious candidate for exhibiting the phenomenon of Bose-Einstein condensation.