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 Tc can be described by a single parameter depending on the intraband and interband impurity scattering rates. Tc 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 ⟨λ⟩. We argue that this transition has strong implications for experiments.