Gap symmetry in KFe ₂As ₂ and the cos 4θ gap component in LiFeAs

We revisit the issue of the gap symmetry in KFe ₂As ₂, which is an Fe pnictide superconductor with only hole pockets. Previous theoretical studies mostly argued for a d-wave gap in KFe ₂As ₂ 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 a scenario for a nodal s-wave superconductivity induced by a nonmagnetic intraband and interband interactions between fermions near hole pockets. The superconducting gap changes sign between the hole pockets and has cos4θ angular dependence and accidental nodes on one or several hole pockets. We argue that strong angle dependence is the consequence of near-degeneracy between interpocket and intrapocket interaction on the hole pockets. We also analyze the cos4θ angular dependence of the gap in other Fe pnictides and compare theoretical results with the photoemission experiments of LiFeAs.