Evidence of nodeless superconductivity in FeSe0.85 from a muon-spin-rotation study of the in-plane magnetic penetration depth

The in-plane magnetic penetration depth lambda(ab) of the iron selenide superconductor with the nominal composition FeSe0.85 was studied by means of muon-spin rotation. The measurements of lambda(-2)(ab)(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 FeSe0.85 contains no nodes.