SUPPRESSED METAL-INSULATOR TRANSITIONS, ENHANCED SUPERCONDUCTIVITY, AND REDUCED ISOTOPE EFFECT DUE TO QUARTIC ANHARMONICITY OF PHONONS IN A PEIERLS-HUBBARD MODEL

A quasi-two-dimensional Peierls-Hubbard model with anharmonic phonons has been studied, so as to clarify the competition between the metallic state and the charge- or spin-density-wave (CDW or SDW) states, as well as the isotope effect on the superconducting (SC) transition temperature T(c) of this system. The anharmonicity is assumed to be a local quartic one, like a 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 quantum fluctuation of phonons and this hard-core-type anharmonicity, the Peierls distortion is excessively suppressed. Thus, the metallic 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 SC state. T(c) due to this anharmonic phonon has no isotope effect, and values from 50 to 100 K. How to observe this local anharmonicity is also discussed in connection with the photoinduced absorption.