Nodal superconductivity coexists with low-moment static magnetism in single-crystalline tetragonal FeS: A muon spin relaxation and rotation study

TL;DR

This study uses muon spin relaxation and rotation measurements on single-crystalline FeS to reveal coexistence of nodal superconductivity with static low-moment magnetism, providing insights into its magnetic state and gap symmetry.

Contribution

It demonstrates the coexistence of nodal superconductivity and static magnetism in FeS and identifies the $s+d$-wave model as the best fit for its gap structure.

Findings

Static magnetism appears below 10 K and coexists with superconductivity.

Superfluid density shows a linear temperature dependence at low temperatures.

Superconducting gap structure is independent of applied magnetic field.

Abstract

We report muon spin relaxation and rotation ($\mu$SR) measurements on hydrothermally-grown single crystals of the tetragonal superconductor~FeS, which help to clarify the controversial magnetic state and superconducting gap symmetry of this compound. $\mu$SR time spectra were obtained from 280~K down to 0.025~K in zero field (ZF) and applied fields up to 20 mT. In ZF the observed loss of initial asymmetry (signal amplitude) and increase of depolarization rate~$\Lambda_\mathrm{ZF}$ below 10~K indicate the onset of static magnetism, which coexists with superconductivity below $T_c$. Transverse-field $\mu$SR yields a muon depolarization rate $\sigma_\mathrm{sc} \propto \lambda_{ab}^{-2}$ that clearly shows a linear dependence at low temperature, consistent with nodal superconductivity. The $s{+}d$-wave model gives the best fit to the observed temperature and field dependencies. The normalized superfluid densities versus normalized temperature for different fields collapse onto the same curve, indicating the superconducting gap structure is independent of field. The $T=0$ in-plane penetration depth $\lambda_{ab}$(0) = 198(3) nm.