Superconductivity in a generalized Hubbard model

We consider a Hubbard model in the square lattice, with a generalized hopping between nearest-neighbor sites for spin up (down), which depends on the total occupation nb of spin down (up) electrons on both sites. We call the hopping parameters t(AA), t(AB), and t(BB) for n(b) = 0, 1 or 2 respectively. Using the Hartree-Fock and Bardeen-Cooper-Schrieffer mean-field approximations to decouple the two-body and three-body interactions, we find that the model exhibits extended s-wave superconductivity in the electron-hole symmetric case t(AB) > t(AA) = t(BB) for small values of the Coulomb repulsion U or small band fillings. For moderate values of U, the antiferromagnetic normal (AFN) state has lower energy. The translationally invariant d-wave superconducting state has always larger energy than the AFN state. (C) 1997 Elsevier Science B.V.