Gap Symmetry in KFe_2As_2 and cos 4θgap component in LiFeAs

英文摘要：

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 4θangular 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 4θangular dependence of the gap in other Fe-pnictides and compare theoretical results with the photoemission experiments of LiFeAs.