Influence of temperature on the impurity-induced s(+/-) -> s(++) transition in the two-band model for Fe-based superconductors

In Fe-based superconducting materials, the s± state with the sign-changing gap in the clean limit could be changed to the sign-conserving s++ state by nonmagnetic impurities. Previous results are obtained for the fixed temperature well below the superconducting critical temperature Tc. We study how the increasing temperature affects the transition between s± and s++ states in the two-band model. The calculations show that the s± → s++ transition appears to be dependent on temperature T, i.e. there exists a narrow range of impurity scattering rates, where the s++ state in dirty superconductor at low temperature is transformed back to the s± state by increasing T. With the nonmagnetic impurity scattering rate increasing, the temperature of such a reverse transition is shifted to Tc, while the s++ state remains solely one for higher degree of disorder.