POLARON AND BIPOLARON COEXISTENCE IN HIGH T-C SUPERCONDUCTIVITY

A model for high T-c superconductivity in which polarons and bipolaron s coexist is studied. The bipolarons are assumed to be in a bound or r esonant state depending on the total charge carrier density and are al lowed to decay into a pair of independent polarons. The critical tempe rature as a function of the total charge carrier density and of the ra tio of the superconducting density to the superconducting charge carri er mass is calculated and compared with the experimental data. It is s hown that the dependence on the total charge carrier density of the bi polaron resonant energy and of the bipolaron-polaron decay probability is important in order to obtain a satisfactory agreement with the exp erimental data.