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

TL;DR

This study reveals highly anisotropic two-gap superconductivity in LaO$_{0.5}$F$_{0.5}$BiS$_2$, with an upper critical field exceeding the Pauli limit due to strong spin-orbit coupling and local inversion symmetry breaking.

Contribution

First detailed measurements of the angular dependence and temperature evolution of upper critical fields in LaO$_{0.5}$F$_{0.5}$BiS$_2$, demonstrating two-gap behavior and Pauli limit violation.

Findings

Anisotropy ratio $H_{c2}^{ ext{parallel}}/H_{c2}^{ ext{perpendicular}}$ reaches ~16 at low temperature.

Upward curvature of $H_{c2}^{ ext{parallel}}(T)$ near $T_c$ indicating two-gap superconductivity.

$H_{c2}^{ ext{parallel}}(0)$ exceeds the Pauli paramagnetic limit, influenced by spin-orbit coupling.

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.