Disorder-induced transition between s± and s++ states in two-band superconductors

We have reexamined the problem of disorder in two-band superconductors, and shown, within the framework of the T-matrix approximation, that the suppression of T-c can be described by a single parameter depending on the intraband and interband impurity scattering rates. T-c is shown to be more robust against nonmagnetic impurities than would be predicted in the trivial extension of Abrikosov-Gor'kov theory. We find a disorder-induced transition from the s(+/-) state to a gapless and then to a fully gapped s(++) state, controlled by a single parameter-the sign of the average coupling constant (lambda). We argue that this transition has strong implications for experiments.