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_{\pm} 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. 
D. V. Efremov, M. M. Korshunov, O. V. Dolgov, A. A. Golubov, and P. J.   Hirschfeld
cond-mat.supr-con cond-mat.str-el