Lattice-instability-enhanced high-temperature superconductivity for the perovskite-family oxides

For a possible origin of the high superconducting transition temperature $T_c$ in the perovskitefamily oxides, we propose a mechanism in which superconductivity is enhanced by a lattice instability. When two nearly degenerate bands overlapping the Fermi energy ${\epsilon }_F$ become unstable against spontaneous splitting because of the presence of electron-phonon coupling, two electrons in the same band attract each other strongly as a reaction of the strong effective repulsion between two electrons in different band. This model is consistent with the observed fact that the high $T_c$ appears near the electronic or structural transition in the mixed perovskite compounds. Guides for a higher $T_c$ are discussed on the basis of the present mechanism.