Single-gap Isotropic $s-$wave Superconductivity in Single Crystals $\text{AuSn}_4$

TL;DR

This study demonstrates that single crystals of AuSn4 exhibit isotropic s-wave superconductivity consistent with BCS theory, with robust transition temperature and superfluid density unaffected by electron irradiation, indicating conventional superconducting behavior.

Contribution

The paper provides experimental evidence that AuSn4 is a conventional isotropic s-wave superconductor with properties matching BCS predictions, including robustness to electron irradiation.

Findings

Superfluid density fits BCS theory across all temperatures.

Superconducting transition temperature remains unchanged after irradiation.

Campbell penetration depth shows no hysteresis, indicating parabolic vortex pinning.

Abstract

London, $\lambda_L (T)$, and Campbell, $\lambda_{C} (T)$, penetration depths were measured in single crystals of a topological superconductor candidate $\text{AuSn}_4$. At low temperatures, $\lambda_L (T)$ is exponentially attenuated and, if fitted with the power law, $\lambda(T) \sim T^n$, gives exponents $n>4$, indistinguishable from the isotropic single $s-$wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, $T_c = 2.40 \pm 0.05\:\text{K}$, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic $s-$wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the $\text{AuSn}_4$ crystals is fully consistent with the isotropic $s-$wave weak-coupling BCS superconductivity.