Anomalous impurity effects in nonadiabatic superconductors

We show that, in contrast with the usual electron-phonon Migdal-Eliashberg theory, the critical temperature T-c of an isotropic s-wave nonadiabatic superconductor is strongly reduced by the presence of diluted nonmagnetic impurities. Our results suggest that the recently observed T-c-suppression driven by disorder in K3C60 (Phys. Rev. B, 55 (1997) 3866) and in Nd2-xCexCuO4-delta (Phys. Rev. B, 58 (1998) 8800) could be explained in terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the isotope effect on T-c has an impurity dependence qualitatively different from the one expected for anisotropic superconductors.