BIPOLARONS

Cooperative properties of self-localized carriers on a lattice are dis cussed and reviewed at a fairly basic level with an emphasis on develo pments of the strong-coupling theory of superconductivity for the inte rpretation of high-T-c superconductors. Small polaron and bipolaron fo rmation is shown to provide a number of new physical phenomena both in the normal and superconducting states. Two mechanisms of superconduct ivity are discussed in detail. The first one arises from the Cooper pa iring of small polarons in momentum space (polaronic superconductivity ) while the second one is due to polaron pairing in real space and ana logous to the superfluidity of He-4 (bipolaronic superconductivity). H ighly non-adiabatic motion of bipolarons results in fundamental differ ences of bipolaronic superconductivity with respect to the scs one inc luding its well known strong-coupling generalization. The review cover s the theoretical development and some experimental results in the pas t decade paying special attention to the physical properties of high-T -c oxides and their explanation with (bi)polarons. Basic properties of charged bosons are also considered.