Polaron models of high-temperature superconductors

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Abstract
The work of Alexandrov and Ranninger has shown that the extrapolation of the BCS theory to strong electron-phonon interaction necessarily involves the formation of small polarons. The resulting theory can be directly applied to the cubic bismuth oxides, in which the bosons are Bi3+, existing both above and below Tc. In the copper oxide materials, the role of the spins in Cu2+ is discussed. A model is proposed in which both spin and dielectric polarons play a role. The model is applied to electrical properties above Tc, for under- and over-doped specimens, and various other phenomena.