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 su perconductor is strongly reduced by the presence of diluted nonmagnetic imp urities. Our results suggest that the recently observed T-c-suppression dri ven 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 nona diabatic electron-phonon coupling. Moreover, we predict that the isotope ef fect on T-c has an impurity dependence qualitatively different from the one expected for anisotropic superconductors.