Disordered superconductors: role of interaction strength

We have considered the half-filled disordered attractive Hubbard model, in which the on-site attraction is switched off on a fraction $f$ of sites, while keeping a finite $U$ on the remaining ones. The configurationally-averaged equal-time pair structure factor has been calculated as a function of temperature, through Quantum Monte Carlo simulations for several $f$ and $U$, and a finite-size scaling {\it ansatz} has been used for the zero-temperature gap. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, $f_c$, which increases with $U$, at least up to the largest values of $U$ we have considered. Also, the normalized zero-temperature gap as a function of $f$, for fixed $U$, shows a maximum near $f_m$, within a range of $U$ values, thus indicating that a small amount of disorder can initially enhance superconductivity. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to ``push'' electrons towards attractive sites.