Theory of thermal conductivity in extended-ss 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 intra- and interband disorder scattering effects, and show that in situations where a low-temperature linear-$T$ 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.Comment: 13 page

Gap structure in the electron-doped Iron-Arsenide Superconductor Ba(Fe0.92Co0.08)2As2: low-temperature specific heat study

We report the field and temperature dependence of the low-temperature specific heat down to 400 mK and in magnetic fields up to 9 T of the electron-doped Ba(Fe0.92Co0.08)2As2 superconductor. Using the phonon specific heat obtained from pure BaFe2As2 we find the normal state Sommerfeld coefficient to be 18 mJ/mol.K^2 and a condensation energy of 1.27 J/mol. The temperature dependence of the electronic specific heat clearly indicate the presence of the low-energy excitations in the system. The magnetic field variation of field-induced specific heat cannot be described by single clean s- or d-wave models. Rather, the data require an anisotropic gap scenario which may or may not have nodes. We discuss the implications of these results.Comment: New Journal of Physics in press, 10 pages, 5 figure