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 depen ds on the total occupation nb of spin down (up) electrons on both site s. 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-Schri effer mean-field approximations to decouple the two-body and three-bod y interactions, we find that the model exhibits extended s-wave superc onductivity in the electron-hole symmetric case t(AB) > t(AA) = t(BB) for small values of the Coulomb repulsion U or small band fillings. Fo r moderate values of U, the antiferromagnetic normal (AFN) state has l ower energy. The translationally invariant d-wave superconducting stat e has always larger energy than the AFN state. (C) 1997 Elsevier Scien ce B.V.