Nodal Superconductivity in Single-crystalline Tetragonal FeS: A Muon Spin Relaxation and Rotation Study*

We report the muon spin relaxation and rotation (μSR) measurements on hydrothermal grown singlecrystalline tetraganol FeS, which is much debated recently on its magnetic state and superconducting gap symmetry. We measured the zero-field relaxation rate λ_ZF down to 0.025 K, which saturates below T_sat ≈ 4 K. The temperature dependence of λ_ZF can be well explained by thermal excitation of electronic fluctuations, demonstrating FeS is a paramagnet. Our transverse field measurements give the in-plane penetration depth λ_ab(0) = 312(5) nm. The linear low-T dependence of depolarization rate σ_sc ∝ λ_ab^-2 reveals the nodal superconductivity in tetragonal FeS. The s+d-waves model gives the best fit to our data at the fields we applied (μ₀H = 7.5 mT, 30
mT, and 75 mT). The normalized superfluid density versus normalized temperature for different field collapse on the same curve, indicating the superconducting gap structure is independent of field up to 75 mT.