Gap Symmetry in KFe_2As_2 and cos 4\theta gap component in LiFeAs

TL;DR

This paper investigates the gap symmetry in KFe2As2, proposing a nodal s-wave superconductivity with cos 4θ angular dependence, and compares theoretical predictions with photoemission experiments in LiFeAs, challenging previous d-wave assumptions.

Contribution

It introduces a scenario for nodal s-wave superconductivity with cos 4θ dependence in KFe2As2, supported by analysis of angle dependence and comparison with experiments.

Findings

Nodal s-wave gap with cos 4θ dependence in KFe2As2

Sign-changing gap between hole pockets due to intra- and inter-band interactions

Comparison of theoretical gap structure with photoemission data in LiFeAs

Abstract

We revisit the issue of the gap symmetry in KFe2As2, which is an Fe-pnictide superconductor with only hole pockets. Previous theoretical studies mostly argued for a d-wave gap in KFe2As2 since transport and thermodynamic measurements point to the presence of the gap nodes. However, a d-wave gap is inconsistent with recent laser-based angle-resolved photoemission measurements. We propose the scenario for a nodal s-wave superconductivity induced by a non-magnetic intra-band and inter-band interactions between fermions near hole pockets. The superconducting gap changes sign between the hole pockets and has cos 4\theta angular dependence and accidental nodes on one or several hole pockets. We argue that strong angle dependence is the consequence of near-degeneracy between inter-pocket and intra-pocket interaction on the hole pockets. We also analyze cos 4\theta angular dependence of the gap in other Fe-pnictides and compare theoretical results with the photoemission experiments of LiFeAs.