From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

TL;DR

This paper investigates the evolution of pairing symmetry in iron-based superconductors, showing a transition from d-wave in KFe2As2 to s-wave in Ba0.6K0.4Fe2As2, highlighting the link between structure and pairing mechanisms.

Contribution

It provides evidence for a change from d-wave to s-wave pairing within the same crystal structure, offering insights into the relationship between band structure and superconducting pairing.

Findings

KFe2As2 exhibits d-wave pairing with vertical line nodes.

Ba0.6K0.4Fe2As2 shows s-wave pairing with higher Tc.

Transition from d-wave to s-wave occurs within the same crystal structure.

Abstract

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.