Multigap nodeless superconductivity in FeSex: Evidence from quasiparticle heat transport

The in-plane thermal conductivity kappa of the iron selenide superconductor FeSex (T-c=8.8 K) was measured down to 120 mK and up to 14.5 T (similar or equal to 3/4H(c2)). In zero field, the residual linear term kappa(0)/T at T -> 0 is only about 16 mu W K-2 cm(-1), less than 4% of its normal-state value. Such a small kappa(0)/T does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of kappa(0)/T in FeSex is very similar to that in NbSe2, a typical multigap s-wave superconductor. We consider our data as strong evidence for multigap nodeless (at least in ab plane) superconductivity in FeSex. This kind of superconducting gap structure may be generic for all Fe-based superconductors.