THERMALIZATION OF EXCITONS IN CU2O

The thermalization of excitons in Cu//2O is investigated by monitoring the lineshape of the phonon-assisted luminescence for different crystal temperatures and excitation conditions. At low temperatures (less than 10 K) the excitons reach thermal equilibrium. At higher temperatures this is the case only for nonresonant excitation, whereas for resonant excitation the exciton distribution is nonthermal. These lineshapes can be explained quantitatively by a theory which takes into account the coupling of excitons to acoustic and optical phonons.