Theory of thermal conductivity in extended-s state superconductors: Application to ferropnictides

Within a two-band model for the recently discovered ferropnictide materials, we calculate the thermal conductivity assuming general superconducting states of A(1g) (s-wave) symmetry, considering both currently popular isotropic sign-changing s states and states with strong anisotropy, including those which manifest nodes or deep minima of the order parameter. We consider both intraband and interband disorder scattering effects, and show that in situations where a low-temperature linear-T term exists in the thermal conductivity, it is not always universal as in d-wave superconductors. We discuss the conditions under which such a term can disappear, as well as how it can be induced by a magnetic field. We compare our results to several recent experiments.