Nodal superconducting gap in tetragonal FeS

Abstract

Low-temperature specific heat has been measured in FeS superconductor with the transition temperature of 4.55 K. It is found that the low-temperature electronic specific heat ${C}_{e}/T$ can be fitted to a linear relation in the low-temperature region, but fails to be described by an exponential relation as expected by an $s$-wave gap. We try fittings to the data with different gap structures and find that a model with one or two nodal gaps can fit the data. Under a magnetic field, the field induced specific heat coefficient $\mathrm{\ensuremath{\Delta}}{\ensuremath{\gamma}}_{e}=[{C}_{e}(H)\ensuremath{-}{C}_{e}(0)]/T$ shows the Volovik relation $\mathrm{\ensuremath{\Delta}}{\ensuremath{\gamma}}_{e}(H)\ensuremath{\propto}\sqrt{H}$, suggesting the presence of nodal gap(s) in this material.