Universal V-shaped temperature-pressure phase diagram in the iron-based superconductors KFe2As2, RbFe2As2, and CsFe2As2

TL;DR

This study reveals a universal V-shaped phase diagram of critical temperature versus pressure in certain iron-based superconductors, indicating a transition in pairing symmetry at a critical pressure, supported by measurements of critical fields and gap structure.

Contribution

The paper uncovers a universal pressure-temperature phase diagram in hole-doped 122 iron superconductors and links it to a change in superconducting pairing symmetry at a critical pressure.

Findings

Identified a universal V-shaped Tc vs P phase diagram in KFe2As2, RbFe2As2, CsFe2As2.

Observed a sudden change in the superconducting gap structure at the critical pressure.

Proposed a transition from d-wave to s+- pairing symmetry across the critical pressure.

Abstract

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a d-wave state favored by high-Q scattering at low pressure changes to a state with s+- symmetry favored by low-Q scattering at high pressure.