THE RENORMALIZATION OF THE ENERGETICAL SPECTRUM AND THE PECULIARITIESOF HIGH-DENSITY PARAEXCITON LUMINESCENCE BAND

The renormalization of the energetical spectrum of paraexcitons in the presence of a Bose-Einstein condensate (BEG) of orthoexcitons or at l east of their saturation is studied. Due to the conversion of a pair o f orthoexcitons into a pair of paraexcitons an absolute instability in the energy spectrum of paraexcitons appears. The appeared paraexciton s can be downscattered by optical phonons Gamma(5)(-) (88 cm(-1)) into a state above the bottom of the paraexciton band. This can be the rea son of the observed peculiarities in the paraexciton luminescence band . On this basis a new explanation of the appearance of an extra peak i n the luminescence spectrum of the uniaxially stressed Cu2O crystal is suggested. It has the same origin as the bump of the luminescence ban d in unstrained crystals. The spectral positions of the extra peak and of the bump differ due to the diminution of the ortho-para splitting in uniaxially stressed crystals. The difference of their widths is due to different densities of the condensed orthoexcitons in stressed and unstrained crystals.