Large polarons, bipolarons and boson-fermion model of superconductivity

Large polaron and bipolaron formation is reviewed with particular emphasis on the screening effects which become important at relatively high carrier density (plasma frequency, omega(p), larger than longitudinal optical frequency, omega(l)). At high density polarons are mobile particles even at very high electron-phonon coupling constant (a much greater than 1) and bipolaron binding energy tends to vanish giving evidence of the existence of metastable bipolaron states. The coexistence of metastable bipolarons (bosons) and polarons (fermions) supports the so-called boson-fermion model of superconductivity. This model can be generalized in order to take into account the dependence on the carrier density given in the polaron-bipolaron framework. The results are in good agreement with the density dependence of the critical temperature exhibited by many high-T-c superconductors.