Origin of pressure induced second superconducting dome in AyFe2-xSe2 [A=K, (Tl,Rb)]

TL;DR

This paper models the pressure-induced transition in AyFe2-xSe2 superconductors, explaining the emergence of a second superconducting phase through Fermi surface topology changes and predicting experimental signatures.

Contribution

It introduces a low-energy effective model linking Fermi surface topology changes to different superconducting phases under pressure.

Findings

Low-pressure phase is a nodeless d-wave superconductor.

High-pressure phase becomes an s+- superconductor with hole pocket formation.

Predicted neutron spin resonance features for each phase.

Abstract

Recent observation of pressure induced second superconducting phase in AyFe2-xSe2 [A=K, (Tl,Rb)] calls for the models of superconductivity that are rich enough to allow for multiple superconducting phases. We propose the model where pressure induced changes in Fermi surface topology lead to appearance of the hole pocket at $\Gamma$ point at high pressure. We develop low-energy effective model, derived from first-principles band-structure calculation, to suggest the phase assignment where low pressure superconducting state with no hole pocket at $\Gamma$ point is a nodeless d-wave state. It evolves into a s+- state at higher pressure when the Fermi surface topology changes and hole pocket appears. We analyze the pairing interactions using five band tight binding fitted band structure and find strong pairing strength dependents on pressure. We also evaluate the energy and momentum dependence of neutron spin resonances in each of the phases as verifiable predictions of our proposal.