CRITICAL VELOCITIES IN EXCITON SUPERFLUIDITY

The presence of exciton phonon interactions is shown to play a key rol e in the exciton superfluidity. We apply the Landau criterion for an e xciton-phonon condensate moving uniformly at zero temperature. It turn s out that there are essentially two critical velocities in the theory . Within the range of these velocities the condensate can exist only a s a bright soliton. The excitation spectrum and differential equations for the wave function of this condensate are derived.