"Excitoner": Stimulated amplification and propagation of excitons' beams

A statistical-thermodynamic theory of the phenomenon of stimulated amplific ation of the population of excitons which lie at the bottom of their lowest -energy band is presented. The experimentally detected "packet" of excitons flowing ballistically is shown to consist of a Schrodinger-Davydov solitar y wave, dressed with a cloud of incoherent excitons. Moreover, a secondary excitation by a c.w. laser beam promotes a Frohlich-Bose-Einstein-like cond ensation, which is responsible for the relevant phenomenon that the lifetim e of the soliton is largely increased with increasing pumping power.