Temperature-Dependent s(+/-) <-> s(++) Transitions in the Multiband Model for Fe-Based Superconductors with Impurities

Shestakov, V. A.; Korshunov, M. M.; Dolgov, O. V. Source: SYMMETRY-BASEL, 10 (8):10.3390/sym10080323 AUG 2018

We study the dependence of the superconducting gaps on both the disorder and the temperature within the two-band model for iron-based materials. In the clean limit, the system is in the $s_{\pm}$ state with sign-changing gaps. Scattering by nonmagnetic impurities leads to the change of the sign of the smaller gap, resulting in a transition from the $s_{\pm}$ to the $s_{+ +}$ state with the sign-preserving gaps. We show here that the transition is temperature-dependent. Thus, there is a line of $s_{\pm} \to s_{+ +}$ transition in the temperature–disorder phase diagram. There is a narrow range of impurity scattering rates, where the disorder-induced $s_{\pm} \to s_{+ +}$ transition occurs at low temperatures, but then the low-temperature $s_{+ +}$ state transforms back to the $s_{\pm}$ state at higher temperatures. With increasing impurity scattering rate, the temperature of such $s_{+ +} \to s_{\pm}$ transition shifts to the critical temperature $T_{c}$ , and only the $s_{+ +}$ state is left for higher amounts of disorder.