Changing nature of superconductivity in tetragonal FeS under pressure

Tetragonal FeS is an iron chalcogenide superconductor with similarities to FeSe but without nematic phase and with weaker electronic correlations. In contrast to pure FeSe, however, FeS shows two superconducting domes under pressure. We use electronic structure and spin fluctuation theory calculations for tetragonal FeS in order to study the nature of the superconducting order parameter [1]. In the random phase approximation, we find a gap function with d-wave symmetry at ambient pressure which is in agreement with several reports of a nodal superconducting order parameter in FeS. We find that, as a function of pressure, the superconducting pairing strength decreases until a Lifshitz transition takes place at 4.6 GPa. As a hole pocket with a large density of states appears at the Lifshitz transition, the gap symmetry is altered to sign-changing s-wave. At the same time the pairing strength is severely enhanced and increases up to a new maximum at 5.5 GPa. Therefore, our calculations explain the occurrence of two superconducting domes in FeS without any structural phase transition.

[1] M. Shimizu, N. Takemori, D. Guterding, H. O. Jeschke, Phys. Rev. Lett. 121, 137001 (2018).