ON TRANSPORT-PROPERTIES OF EXCITON-PHONON CONDENSATE IN CU2O

A moving condensate of para-excitons in a SD Cu2O crystal turns out to be spatially inhomogeneous in the direction of motion, and the regist ered velocities of coherent exciton packets are approximately equal to the longitudinal sound speed of the crystal. We explore a simple theo retical model to describe such properties of the Bose condensed excito ns. Taking into account the exciton - phonon interaction and introduci ng the coherent phonon part of the moving condensate, we derive the dy namic equations for the exciton - phonon condensate. Within the Bose a pproximation for excitons, we discuss the conditions for the moving co ndensate to appear in the crystal. We calculate the condensate wave fu nction and energy and a collective excitation spectrum in the semiclas sical approximation. The stability conditions of the moving condensate are analyzed as well.