Abstract
A statistical-thermodynamic theory of the phenomenon of stimulated amplification 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 Schrödinger-Davydov solitary wave, dressed with a cloud of incoherent excitons. Moreover, a secondary excitation by a c.w. laser beam promotes a Fröhlich-Bose-Einstein–like condensation, which is responsible for the relevant phenomenon that the lifetime of the soliton is largely increased with increasing pumping power.