POLARON AND BIPOLARON COEXISTENCE IN HIGH T-C SUPERCONDUCTIVITY

A model for high T-c superconductivity in which polarons and bipolarons coexist is studied. The bipolarons are assumed to be in a bound or resonant state depending on the total charge carrier density and are allowed to decay into a pair of independent polarons. The critical temperature as a function of the total charge carrier density and of the ratio of the superconducting density to the superconducting charge carrier mass is calculated and compared with the experimental data. It is shown that the dependence on the total charge carrier density of the bipolaron resonant energy and of the bipolaron-polaron decay probability is important in order to obtain a satisfactory agreement with the experimental data.