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

Abstract

Ambient-pressure-grown LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ with a superconducting transition temperature $T_c\sim$3K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic field direction varying between the crystalline $c$-axis and the $ab$-plane, we present the first datasets displaying the temperature dependence of the out-of-plane upper critical field $H_{c2}^{\perp }(T)$, the in-plane upper critical field $H_{c2}^{\parallel }(T)$, as well as the angular dependence of $H_{c2}$ at fixed temperatures for ambient-pressure-grown LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ single crystals. The anisotropy of the superconductivity, $H_{c2}^{\parallel }/H_{c2}^{\perp }$, reaches $\sim$16 on approaching 0 K, but it decreases significantly near $T_c$. A pronounced upward curvature of $H_{c2}^{\parallel }(T)$ is observed near $T_c$, which we analyze using a two-gap model. Moreover, $H_{c2}^{\parallel }(0)$ is found to exceed the Pauli paramagnetic limit, which can be understood by considering the strong spin-orbit coupling associated with Bi as well as the breaking of the local inversion symmetry at the electronically active BiS${}_2$ bilayers. Hence, LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ with a centrosymmetric lattice structure is a unique platform to explore the physics associated with local parity violation in the bulk crystal.

6 pages, 4 figures