Anisotropic two-gap superconductivity and the absence of Pauli paramagnetic limit in single crystalline LaO$_{0.5}$F$_{0.5}$BiS$_2$

摘要

Ambient-pressure-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$ with a superconductingtransition temperature $T_{c}~\sim$3 K has a two-dimensional-like Fermi surfaceaccording to band structure calculations, suggesting a highly anisotropicnormal state. By a series of electrical resistivity measurements with amagnetic field direction varying between the crystalline $c$-axis and the$ab$-plane, we present the first datasets displaying the temperature dependenceof the out-of-plane upper critical field $H_{c2}^{\perp}(T)$, the in-planeupper critical field $H_{c2}^{\parallel}(T)$, as well as the angular dependenceof $H_{c2}$ at fixed temperatures for ambient-pressure-grownLaO$_{0.5}$F$_{0.5}$BiS$_2$ single crystals. The anisotropy of thesuperconductivity, $H_{c2}^{\parallel}/H_{c2}^{\perp}$, reaches $\sim$16 onapproaching 0 K, but it decreases significantly near $T_{c}$. A pronouncedupward curvature of $H_{c2}^{\parallel}(T)$ is observed near $T_{c}$, which weanalyze using a two-gap model. Moreover, $H_{c2}^{\parallel}(0)$ is found toexceed the Pauli paramagnetic limit, which can be understood by considering thestrong spin-orbit coupling associated with Bi as well as the breaking of thelocal inversion symmetry at the electronically active BiS$_2$ bilayers.