Two-dome superconductivity in FeS induced by a Lifshitz transition

TL;DR

This study explains the emergence of two superconducting domes in FeS under pressure by analyzing electronic structure changes and a Lifshitz transition, revealing a switch from d-wave to s-wave symmetry and enhanced pairing.

Contribution

It provides a theoretical explanation for the pressure-induced two-dome superconductivity in FeS, highlighting the role of a Lifshitz transition and changes in gap symmetry.

Findings

Superconductivity in FeS shows two domes under pressure.

A Lifshitz transition at 4.6 GPa alters gap symmetry from d-wave to s-wave.

Pairing strength is significantly enhanced after the Lifshitz transition.

Abstract

Among iron chalcogenide superconductors, FeS can be viewed as a simple, highly compressed relative of FeSe without nematic phase and with weaker electronic correlations. Under pressure, however, the superconductivity of stoichiometric FeS disappears and reappears, forming two domes. We perform electronic structure and spin fluctuation theory calculations for tetragonal FeS in order to analyze the nature of the superconducting order parameter. In the random phase approximation we find a gap function with d-wave symmetry at ambient pressure, in agreement with several reports of a nodal superconducting order parameter in FeS. Our calculations show 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 naturally explain the occurrence of two superconducting domes in FeS.