Pseudospin-Triplet Pairing in Iron-Chalcogenide Superconductors

TL;DR

This paper investigates a new type of pairing in iron-chalcogenide superconductors involving pseudospin-triplet states, explaining gap anisotropy and coexistence with nematicity through a weak coupling framework.

Contribution

It introduces a weak coupling criterion for pseudospin-triplet pairing, revealing its mixing with s-wave pairing and implications for gap structures and nematicity in iron-based superconductors.

Findings

Pseudospin-triplet pairing can mix with s-wave pairing within the same symmetry.

Gap anisotropy is enhanced by nematicity and can explain experimental observations.

Coexistence of nematicity and superconductivity is supported by energetic stabilization of pseudospin-triplet pairing.

Abstract

We study superconductivity of electron systems with both spin and pseudospin-1/2 degrees of freedom. By solving linearized gap equations, we derive a weak coupling criterion for the even-parity spin-singlet pseudospin-triplet pairing. It can generally mix with the on-site s-wave pairing since both of them belong to the same symmetry representation ($A_{1g}$) and their mixture could naturally give rise to anisotropic intra-band pairing gap functions with or without nodes. This may directly explain why some of the iron-chalcogenide superconductors are fully gapped (e.g. FeSe thin film) and some have nodes (e.g. LaFePO and LiFeP). We also find that the anisotropy of gap functions can be enhanced when the principal rotation symmetry is spontaneously broken in the normal state such as nematicity, and the energetic stabilization of pseudospin-triplet pairings indicates the coexistence of nematicity and superconductivity. This could be potentially applied to bulk FeSe, where gap anisotropy has been experimentally observed.