Upper critical field study of LaO_0.5F_0.5BiS₂ single crystals

The anisotropy between the in-plane upper critical field H_c2^//ab and the out-of-plane upper critical field H_c2^⊥ab in as-grown LaO_0.5F_0.5BiS₂ has been investigated in polycrystalline samples for years, but studies on single crystals, which are more ideal, are absent. Here the upper critical field investigations of LaO_0.5F_0.5BiS₂ single crystals under various field angles via transport measurements are presented. The anisotropic factor H_c2^//ab/H_c2^⊥ab reaches ~16 at zero-temperature limit. The temperature dependence of H_c2^//ab(T) shows a pronounced upward curvature near the critical temperature T_c, which can be described by a two-gap model in dirty limit. These features suggest that LaO_0.5F_0.5BiS₂ is a highly anisotropic two-gap superconductor. Furthermore, H_c2^//ab(0) exceeds Pauli paramagnetic limit. The strong spin-orbit coupling associated with Bi and the breaking of the local inversion symmetry at the electronically active BiS₂ bilayers are proposed to explain this behavior.