Nodal superconductivity in FeS: Evidence from quasiparticle heat transport

We report low-temperature heat transport measurements on superconducting iron sulfide FeS with $T_c\approx$ 5 K, which has the same crystal structure and similar electronic band structure to the superconducting iron selenide FeSe. In zero magnetic field, a significant residual linear term ${\kappa }_0\mathrm{/}T$ is observed. At low field, ${\kappa }_0\mathrm{/}T$ increases rapidly with the increase of field. These results provide strong evidence for nodal superconducting gap in FeS. We compare it with the sister compound FeSe, and other iron-based superconductors with nodal gap.