Large-radius bipolaron and the polaron-polaron interaction

Research on the polaron-polaron interaction and the theory of large-radius bipolarons are reviewed. The difference between the two-center and one-center continuum bipolaron models in isotropic and anisotropic crystals is discussed. It is shown that the inclusion of electron electron correlations can significantly reduce the bipolaron and D--center energies as well as the energies of exchange-bound pairs of shallow hydrogen-like centers. The two-center bipolaron configuration corresponds to a shallow secondary minimum and is unstable. The phonon-mediated exchange interaction between Pekar polarons has an antiferromagnetic nature and exceeds the ferromagnetic interaction due to the Coulomb interaction of electrons localized in polaron potential wells. The possibility that the super-fluidity of bipolarons can give rise to high-temperature superconductivity is discussed and problems related to the Wigner crystallization of a polaron gas are examined.