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 inter- and intra-site 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.