The interrelation between instability and a high superconducting transition temperature Tc is argued theoretically in order to construct a model for the origin of the high Tc in the perovskite-type oxides. It is shown that when two nearly degenerate bands overlapping on the Fermi energy εF become unstable against spontaneous splitting to give rise to a charge redistribution, effective interactions between two electrons in the same bands become attractive, and the attractive interactions are strongly enhanced by increasing the degree of instability. One cause of this instability is the electron–phonon coupling, which results in lattice-instability-enhanced superconductivity; another cause is the Coulomb interaction between electrons, which results in electron-instability-caused superconductivity. The latter mechanism is successfully applied to the perovskite-type high-Tc superconductors. Some guidelines for obtaining a high Tc are presented on the basis of the present idea.
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