S-wave superconductivity with strong local repulsion

We study the existence of extended s-wave superconductivity in a single band Hubbard model, in the large U limit and with a nearest neighbour attraction. Using a Green's function method we obtained a satisfactory pair wave function, where on site amplitude vanishes for large U. But the attraction Vs must exceed a critical value Vsc to make this superconductivity appear. However, Vsc can be suppressed with the inclusion of second neighbour hopping in the Hamiltonian. A result of this paper is that no superconductivity occurs in the simple Hubbard model in the large U limit, because the s-channel attraction is lower than Vsc.