Details of the disorder-induced transition between s± and s++ states in the two-band model for Fe-based superconductors

Irradiation of superconductors with different particles is one of many ways to investigate the effects of disorder. Here we study the disorder-induced transition between s(+/-) and s(++) states in the two-band model for Fe-based superconductors with nonmagnetic impurities. Specifically, we investigate the important question of whether the superconducting gaps during the transition change smoothly or abruptly. We show that the behavior can be of either type and is controlled by the ratio of intraband to interband impurity scattering potentials, and by a parameter sigma, that represents scattering strength and ranges from zero (Born approximation) to one (unitary limit). For the pure interband scattering potential and the scattering strength sigma less than or similar to 0.11, the s(+/-) -> s(++) transition is accompanied by steep changes in the gaps, while for larger values of s, the gaps change smoothly. The behavior of the gaps is characterized by steep changes at low temperatures, T < 0.1T(c0) with T-c0 being the critical temperature in the clean limit, otherwise it changes gradually. The critical temperature T-c is always a smooth function of the scattering rate in spite of the steep changes in the behavior of the gaps.