We consider a superconductor-insulator-superconductor (SIS) junction in which the tunnelling through the insulating barrier is dominated by a localized ``negative U'' center. We show that the IcR product of the junction depends sensitively on the spectrum of impurity states, and in near resonant condiditions exhibits an anomalously large IcR product which can exceed the famous Ambegoakar-Baratoff limit by an arbitrarily large factor. The analysis is extended to problems in which there is an array of negative U centers in the junction. We also discuss general reasons to expect significant violations of the optical conductivity sum rule in most SIS junctions and of the Ambegoakar-Baratoff result when the superconductors emerge from a non-Fermi liquid normal state.
@article{arxiv.cond-mat/0109129,
title = {Josephson tunnelling spectroscopy of negative U centers},
author = {Vadim Oganesyan and Steven Kivelson and Theodore Geballe and Boris Moyzhes},
journal= {arXiv preprint arXiv:cond-mat/0109129},
year = {2016}
}