Nodal Superconducting Gap in ${\hat{I}}^2$-FeS

Abstract

Low temperature specific heat has been measured in superconductor ${\hat{I}}^2$-FeS with T${}_c$ = 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 $\hat{I}”{\hat{I}}^3$=[C${}_e$(H)-C${}_e$(0)]/T shows the Volovik relation $\hat{I}”{\hat{I}}_e^3(H)\propto \sqrt{H}$, suggesting the presence of nodal gap(s) in this material.

6 pages, 5 figures