Excessively suppressed metal-insulator transitions, enhanced superconductivity and reduced isotope effect by hard-core type anharmonicity of phonons in peierls-hubbard model

Abstract

A quasi two-dimensional Peierls-Hubbard model with anharmonic phonons has been studied, so as to clarify the competition between the superconducting(SP) stste and the charge- or spin-density wave (CDW or SDW) state, as well as the isotope effect on the SP transition temperature T C of this system. The anharmonicity is assumed not to be a double well, but to be a local quartic one, like an interionic hard-core repulsion. This theory is based on the mean field approximation for electrons and a variational method for phonons. Because of an interplay between the quatum fluctuation of phonons and this hard-core type anharmonicity, the Peierls distortian is excessively suppressed. Thus, the SP state becomes more stable than the CDW, even when the electron-phonon coupling is very strong. This interplay is shown to suppress the SDW, too, and relatively enhances the SP state. T C due to this anharmonic phonon has no isotope effect, and amounts to around 50K≈100K.