  As the simplest iron-based superconductor, FeSe forms a tetragonal structure
with transition temperature Tc ~ 8 K. With assistance of pressure, or other
techniques, Tc can be greatly enhanced, even to above liquid nitrogen
temperature. The newly discovered superconducting tetragonal FeS (Tc ~ 4.5 K),
a sulfide counterpart of FeSe, promotes us on its high pressure investigation.
The transport and structure evolution of FeS with pressure have been studied. A
rapid suppression of Tc and vanishing of superconductivity at 4.0 GPa are
observed, followed by a second superconducting dome with a 30% enhancement in
maximum Tc. 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.
