Pairing instabilities of the extended Hubbard model for Cu-O–based superconductors

We use weak-coupling methods to calculate the effective interaction between quasiparticles induced by charge and spin fluctuations in a three-band model for the copper-oxide superconductors with both intersite and intrasite repulsion. The instabilities to superconducting states of different symmetries are calculated; besides the d-wave superconductivity near the antiferromagnetic instability, we find that extended s-wave superconductivity is favored by a charge transfer resonance close to a valence instability. A phase diagram similar to experiments is obtained.