Multi-gap nodeless superconductivity in iron selenide FeSe${}_x$: evident from quasiparticle heat transport

Abstract

The in-plane thermal conductivity $\hat{I}º$ of the iron selenide superconductor FeSe${}_x$ ($T_c$ = 8.8 K) were measured down to 120 mK and up to 14.5 T ($\simeq 3/4H_{c2}$). In zero field, the residual linear term $\hat{I}{º}_0/T$ at $T\to 0$ is only about 16 $\hat{I}¼$W K${}^{-2}$ cm${}^{-1}$, less than 4% of its normal state value. Such a small $\hat{I}{º}_0/T$ does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of $\hat{I}{º}_0/T$ in FeSe${}_x$ is very similar to that in NbSe${}_2$, a typical multi-gap s-wave superconductor. We consider our data as strong evidence for multi-gap nodeless superconductivity in FeSe${}_x$. This kind of superconducting gap structure may be generic for all Fe-based superconductors.

5 pages, 4 figures