Anomalous impurity effects in nonadiabatic superconductors

TL;DR

This paper demonstrates that in nonadiabatic superconductors, impurities significantly suppress the critical temperature, contrasting with traditional theories, and explains experimental observations in specific materials.

Contribution

It introduces a nonadiabatic theory showing impurity-induced Tc suppression and predicts a distinct impurity dependence of the isotope effect.

Findings

Impurities strongly reduce Tc in nonadiabatic superconductors.

The theory explains Tc suppression observed in K3C60 and Nd(2-x)CexCuO(4-delta).

Isotope effect on Tc depends on impurities differently than in anisotropic superconductors.

Abstract

We show that, in contrast with the usual electron-phonon Migdal-Eliashberg theory, the critical temperature Tc of an isotropic s-wave nonadiabatic superconductor is strongly reduced by the presence of diluted non-magnetic impurities. Our results suggest that the recently observed Tc-suppression driven by disorder in K3C60 [Phys. Rev. B vol.55, 3866 (1997)] and in Nd(2-x)CexCuO(4-delta) [Phys. Rev. B vol.58, 8800 (1998)] could be explained in terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the isotope effect on Tc has an impurity dependence qualitatively different from the one expected for anisotropic superconductors.