Pairing interaction mediated by the Cu-O charge-transfer oscillations associated with LO phonons in oxide superconductors and their high-Tcsuperconductivity

The mechanism for the high-$T_c$ superconductivity in the oxide superconductors is theoretically investigated. The interplay of the Cu-O electron transfer and the Coulomb interaction acting on electrons produces large charge fluctuations with low frequencies in the oxides. The LO phonons strongly interact with the charge fluctuations and generate the Cu-O charge-transfer oscillations with frequencies close to the LO phonons. The amplitudes of the oscillations are extremely large. The experimental results of electron-energy-loss spectroscopy and neutron scattering provide evidence to show the large amplitude of the charge oscillations in the oxides. The interaction mediated by the charge oscillations yields a strong attractive force acting between electrons. We suggest that the attractive force is responsible for the high-$T_c$ superconductivity in the oxides.