Charge-fluctuation mechanism of high-Tcsuperconductivity and the isotope effect in oxide superconductors

In connection with high-$T_c$ superconductivity, superconductivity in a system of Fermi particles interacting via a pairing interaction mediated by charge fluctuation and LO phonons is theoretically studied. The anomaly observed in the LO phonon dispersion of the oxides suggests that the electronic dielectric constants with certain wave numbers are negative due to the strong Coulomb correlation. The superconducting transition temperature $T_c$ of the system calculated by an Eliashberg-type equation is unusually high and the isotope effect for $T_c$ is very weak.