Hund interaction, spin-orbit coupling and the mechanism of superconductivity in strongly hole-doped iron pnictides

TL;DR

This paper proposes a new s-wave pairing mechanism in strongly hole-doped iron pnictides, driven by Hund's coupling and spin-orbit interaction, explaining experimental observations in KFe2As2.

Contribution

It introduces a novel pairing mechanism involving Hund's coupling and spin-orbit effects specific to strongly hole-doped Fe-based superconductors.

Findings

Superconductivity arises when Hund's coupling exceeds inter-orbital repulsion.

Transition temperature increases as hole doping decreases.

Predicted pairing gaps are anisotropic with possible accidental nodes.

Abstract

We present a novel mechanism of s-wave pairing in Fe-based superconductors. The mechanism involves holes near dxz/dyz pockets only and is applicable primarily to strongly hole doped materials. We argue that as long as the renormalized Hund's coupling J exceeds the renormalized inter-orbital Hubbard repulsion U', any finite spin-orbit coupling gives rise to s-wave superconductivity. This holds even at weak coupling and regardless of the strength of the intra-orbital Hubbard repulsion U. The transition temperature grows as the hole density decreases. The pairing gaps are four-fold symmetric, but anisotropic, with the possibility of eight accidental nodes along the larger pocket. The resulting state is consistent with the experiments on KFe2As2.