Muon-spin rotation study of the in-plane magnetic penetration depth of   FeSe_0.85: evidence for nodeless superconductivity
  The in-plane magnetic penetration depth \lambda_{ab} of the iron selenide superconductor with the nominal composition FeSe_0.85 was studied by means of muon-spin rotation. The measurements of \lambda_{ab}^{-2}(T) are inconsistent with the presence of nodes in the gap as well as with a simple isotropic s-wave type of the order parameter, but can be equally well described within a two-gap (s+s) and an anisotropic s-wave gap picture. This implies that the superconducting energy gap in FeSe_0.85 contains no nodes. 
R. Khasanov, K. Conder, E. Pomjakushina, A. Amato, C. Baines, Z.   Bukowski, J. Karpinski, S. Katrych, H.-H. Klauss, H. Luet...