Magnetic impurities in the two-band model for Fe-based superconductors

Superconductors with different gap symmetries behave differently being subject to the disorder. It is especially important to determining this exact behavior in the Fe-based materials where both the order parameter symmetry and the mechanism of superconductivity are unknown. Here we analyze how the magnetic disorder affects the low-energy properties of the two-band $s_\pm$ and $s_{++}$ models. In a general case, $T_c$ is suppressed approximately following the Abrikosov-Gor'kov trend. There are, however, few exceptional cases with the saturation of $T_c$ for the finite amount of impurities: 1) $s_\pm$ superconductor with the purely interband impurity scattering potential or with the unitary impurities, 2) $s_{++}$ state with the purely interband scattering. We show that the latter unusual behavior is due to the $s_{++} \to s_\pm$ transition. Similar to the case of non-magnetic impurities in a two-band superconductor, the transition occurs depending on the sign of the average coupling constant $\langle \lambda \rangle$.