The in-plane thermal conductivity $\kappa$ of overdoped FeAs-based superconductor ${\text{BaFe}}_{1.73}{\text{Co}}_{0.27}{\text{As}}_2$ $\left(T_c=8.1\text{K}\right)$ 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 ${\kappa }_0/T\left(H\right)$ may reveal an exotic superconducting gap structure in overdoped ${\text{BaFe}}_{1.73}{\text{Co}}_{0.27}{\text{As}}_2$: the vanishing hole $\left(\beta \right)$ 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.

• Received 15 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.094520