Observation of two distinct superconducting domes under pressure in   tetragonal FeS
  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. 
J. Zhang, F. L. Liu, T. P. Ying, N. N. Li, Y. Xu, L. P. He, X. C.   Hong, Y. J. Yu, M. X. Wang, J. Shen, W. G. Yang, S. Y. Li
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el