Charge-fluctuation-mediated superconductivity in the d-p model - Strong-coupling calculation

Charge-fluctuation-mediated superconductivity in the simplest d-p model is investigated with the help of the auxiliary particle method together with the 1/N expansion theory. Transition temperatures, T-c, for various pairing symmetries are calculated by solving the linearized Eliashberg equation retaining full momentum and frequency dependences of the vertex functions, which are repulsive almost over the entire momentum and frequency space. Strong frequency dependences of the vertex functions due to the charge fluctuation turn out to be essential to realize such superconducting states. The extended s symmetric pairing composed of mainly on-site and nearest-neighbor pairing has the highest T-c similar or equal to 85 K at half-filling with a rather small charge-transfer energy where the system is metallic in contrast to the actual high-T-c materials.