POLARON MODELS OF HIGH-TEMPERATURE SUPERCONDUCTORS

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 T(c). 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 T(c), for under- and over-doped specimens, and various other phenomena.