Observation of two superconducting domes under pressure in tetragonal FeS

We investigate the evolution of superconductivity and structure with pressure for the new superconductor FeS (T _c ≈ 4.5 K), a sulfide counterpart of FeSe. 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.