Thermal conductivity of overdoped BaFe1.73Co0.27As2 single crystal: Evidence for nodeless multiple superconducting gaps and interband interactions

The in-plane thermal conductivity kappa of overdoped FeAs-based superconductor BaFe1.73Co0.27As2 (T-c=8.1 K) single crystal was measured down to 80 mK. In zero field, the residual linear term kappa(0)/T is negligible, suggesting a nodeless superconducting gap in the ab plane. In magnetic field, kappa(0)/T increases rapidly, very different from that of conventional s-wave superconductors. This anomalous kappa(0)/T(H) may reveal an exotic superconducting gap structure in overdoped BaFe1.73Co0.27As2: the vanishing hole (beta) pocket has a much larger gap than the electron (gamma and delta) pockets which contain most of the carriers. Such an exotic gap structure is an evidence for superconducting state induced by interband interactions, in which the band with the smaller density of states has a larger gap.