Coexistence of low moment magnetism and superconductivity in tetragonal FeS and suppression of $T_\mathrm{c}$ under pressure

TL;DR

This study reveals that tetragonal FeS exhibits coexisting low-moment magnetism and superconductivity, with superconducting properties suppressed under pressure, providing insights into its electronic and magnetic behavior.

Contribution

First local probe measurements showing coexistence of magnetism and superconductivity in tetragonal FeS, and analysis of pressure effects on $T_c$.

Findings

Low moment disordered magnetism coexists with superconductivity in FeS.

Superconducting gap is fully gapped and consistent with a two-gap s-wave model.

Superconducting $T_c$ decreases continuously under hydrostatic pressure.

Abstract

We report local probe ($\mu$SR) measurements on the recently discovered tetragonal FeS superconductor which has been predicted to be electronically very similar to superconducting FeSe. Most remarkably, we find that low moment ($10^{-2}-10^{-3}\mu_\mathrm{B})$ disordered magnetism with a transition temperature of $T_\mathrm{N}\approx 20$ K microscopically coexists with bulk superconductivity below $T_\mathrm{c}=4.3(1)$ K. From transverse field $\mu$SR we obtain an in-plane penetration depth $\lambda_\mathrm{ab}(0)=223(2)$ nm for FeS. The temperature dependence of the corresponding superfluid density $\lambda_\mathrm{ab}^{-2}(T)$ indicates a fully gapped superconducting state and is consistent with a two gap s-wave model. Additionally, we find that the superconducting $T_\mathrm{c}$ of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa.