Off-Diagonal Nonmagnetic Disorder in s- and d-Wave Superconductors

The previously developed approach to study off-diagonal disorder has been extended and applied to both s- and d-wave superconductors. The derived equations allow the self-consistent solution of the problem. For a special case of negative U centres embedded in non-superconducting host we have calculated their critical concentration for the appearance of superconductivity and found it equal to≈0.4. For x>x$\text{}_{cr}$ the order parameter continuously increases to its maximal value at x=1.