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

Abstract

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