CHARGE-FLUCTUATION-MEDIATED SUPERCONDUCTIVITY IN THE D-P MODEL - CRITICAL-STUDY OF THE RETARDATION EFFECTS

Charge-fluctuation-mediated superconductivity in the simplest d-p model is investigated using the auxiliary boson method together with the 1/N-expansion theory. The transition temperature T-c is determined by solving the Eliashberg equation derived for the pairing interaction including the full momentum and frequency dependence. The pairing state with the highest T-c has an extended s-symmetry described mainly by the on-site and the nearest-neighbor pairings. While the pairing interaction is repulsive over almost the entire momentum space, the frequency dependence, i.e., the retardation effect, is essential to realize such a superconducting state. The order of magnitude of T-c is estimated to be less than or similar to 230 K with a reasonable set of the parameters, which means that the charge fluctuation plays a crucial role in the system.