Change of pairing symmetry in the iron-based superconductor KFe2As2

TL;DR

This study demonstrates a pressure-induced change in pairing symmetry in KFe2As2, shifting from d-wave to s+- state, supporting the role of spin fluctuations in its superconductivity.

Contribution

It provides experimental evidence of a pairing symmetry transition under pressure in KFe2As2, revealing a possible s + id state and advancing understanding of pairing mechanisms.

Findings

Pressure causes a switch from d-wave to s+- pairing symmetry.

Hall coefficient remains constant across the transition, indicating unchanged Fermi surface.

Transition likely involves an s + id state breaking time-reversal symmetry.

Abstract

The pairing mechanism in iron-based superconductors is the subject of ongoing debate. Proximity to an antiferromagnetic phase suggests that pairing is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+-), while the latter yield a standard s-wave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a change of pairing state. The critical temperature Tc decreases with pressure initially, and then suddenly increases, above a critical pressure Pc. The constancy of the Hall coefficient through Pc rules out a change in the Fermi surface. There is compelling evidence that the pairing state below Pc is d-wave, from bulk measurements at ambient pressure. Above Pc, the high sensitivity to disorder argues for a particular kind of s+- state. The change from d-wave to s-wave is likely to proceed via an unusual s + id state that breaks time-reversal symmetry. The proximity of two distinct pairing states found here experimentally is natural given the near degeneracy of d-wave and s+- states found theoretically. These findings make a compelling case for spin-fluctuation-mediated superconductivity in this key iron-arsenide material.