Dipole plasma oscillations and Bose-Einstein condensation of excitons

Dipole plasma oscillations in molecular crystals1 under conditions of Bose–Einstein condensation of excitons are investigated by means of the double-time temperature Green’s function method. Unlike the linear dispersion law of dipole plasma oscillations, obtained without allowance for Bose-Einstein condensation of excitons,1 in the present case the spectrum of dipole plasma oscillations has a gap, similarly to plasmons in the electron gas. The gap is proportional to the exciton concentration in the condensate, and corresponds to a frequency of the order of 10 GHz. This effect can be detected by measuring the resonance absorption of VHF-oscillations in molecular crystals at low temperatures in the laser pumping regime.