Evidence of nodeless superconductivity inFeSe0.85from a muon-spin-rotation study of the in-plane magnetic penetration depth

Abstract

The in-plane magnetic penetration depth ${\ensuremath{\lambda}}_{ab}$ of the iron selenide superconductor with the nominal composition ${\text{FeSe}}_{0.85}$ was studied by means of muon-spin rotation. The measurements of ${\ensuremath{\lambda}}_{ab}^{\ensuremath{-}2}(T)$ are inconsistent with a simple isotropic $s$-wave type of the order parameter but are rather in favor of two-gap $(s+s)$ and anisotropic $s$-wave order parameter symmetries, thus implying that the superconducting energy gap in ${\text{FeSe}}_{0.85}$ contains no nodes.