Nodal superconductivity in FeS: Evidence from quasiparticle heat transport

We report low-temperature heat transport measurements on superconducting iron sulfide FeS with Tc≈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 κ0/T is observed. At low field, κ0/T increases rapidly with increasing field. These results suggest a nodal superconducting gap in FeS. We compare it with the sister compound FeSe and other iron-based superconductors with nodal gaps.