RELAXATION KINETICS OF A LOW-DENSITY EXCITON GAS IN CU2O

The thermalization of a low-density gas of excitons in Cu2O is analyze d within the phonon-assisted Boltzmann kinetics in the presence of a w eak interaction with the light field. For optically quadrupole-allowed orthoexcitons, the thermalization kinetics is formulated in terms of the polariton picture. A ''bottleneck'' relaxation along the lower pol ariton branch strongly dominates over the accumulation of orthoexciton s in the energy minimum of the upper polariton dispersion branch. For paraexcitons, which are strictly forbidden in resonant optical transit ions, a rather slow nonexponential occupation kinetics of the ground-s tate mode at temperature T less than or equal to T-c (T-c is the conde nsation critical temperature) prevents the development of a steady-sta te Bose-Einstein condensate within the optical-phonon-assisted radiati ve lifetime of paraexcitons. While at T less than or equal to T-c the high-energy orthoexcitons or paraexcitons quasiequilibrate into the Pl anck distribution (the effective chemical potential is equal to zero), the low-energy excitons in a narrow spectral band (<100 mu eV) are st ill far from equilibrium. This provides us with an alternative interpr etation of the recent reports on the observation of quasi-steady-state Bose-Einstein condensation of excitons in Cu2O. [S0163-1829(98)03816- 8].