Enhancement of thermal conductivity in the superconducting state of Co-doped BaFe2As2

We report the thermal conductivity in Co-doped BaFe2As2 at under-, optimal-, and over-doping levels. We find the enhancement of the thermal conductivity divided by temperature kappa/T in the superconducting state at all doping levels, which is strikingly similar to those observed in the strongly correlated superconductors, such as high-T-c cuprates and heavy fermion systems. We also find that the enhancement of kappa/T is strongly suppressed by applying magnetic fields. The analysis based on the vortex-scattering model unveils that the enhancement of kappa/T originates from the steep increase of the quasi-particle mean-free path possibly due to the reduction of the inelastic scattering in the superconducting state. Strong enhancement of kappa(e)/T of over-doping level may suggest that quasiparticle population still remains even at low temperatures.