The gap structure and temperature-pressure phase diagram of FeS superconductor

We report low-temperature heat transport measurements on both single crystal and foil of iron sulfide FeS ($T_c \approx$ 4.5 K), a sulfide counterpart of FeSe. In zero magnetic field, a significant residual linear term $\kappa_0/T$ is observed. At low field, $\kappa_0/T$ increases rapidly with the increase of field. These results suggest a nodal superconducting gap in FeS. The origin of this nodal superconductivity in FeS is discussed, by comparing with other iron-based superconductors with nodal gap. Further, we perform high-pressure measurements on the FeS single crystals. A rapid suppression of $T_c$ and vanishing of superconductivity at 4.0 GPa are observed, followed by a second superconducting dome from 5.0 to 22.3 GPa with a 30\% enhancement in maximum $T_c$. An onsite tetragonal to hexagonal phase transition occurs around 7.0 GPa, followed by a broad pressure range of phase coexistence. The residual deformed tetragonal phase is considered as the source of second superconducting dome. The observation of two superconducting domes in iron-based superconductors poses great challenges for understanding their pairing mechanism. \bf Reference Phys. Rev. B {\bf 94}, 100504(R) (2016). arXiv:1604.05254.