Classification of Symmetry Derived Pairing at M Point in FeSe

TL;DR

This paper investigates the possible pairing symmetries in FeSe superconductors, identifying three fully gapped states consistent with experimental data, and analyzes the underlying pairing mechanisms influenced by electronic interactions and spin-orbit coupling.

Contribution

It provides a symmetry-based classification of pairing states in FeSe and insights into the microscopic interactions affecting these states.

Findings

Identifies three possible pairing symmetries: s-wave, d-wave, and helical p-wave.

All states are fully gapped with minima above the Fermi surface, matching experimental observations.

Highlights the roles of electronic interactions and spin-orbit coupling in pairing mechanisms.

Abstract

Using the constraints imposed by the crystalline symmetry of FeSe and the experimentally observed phenomenology, we analyze the possible pairing symmetry of the superconducting order parameter focusing on intercalated and monolayer FeSe compounds. Such analysis leads to three possible pairing symmetry states -- s-wave, d-wave, and helical p-wave. Despite the differences in the pairing symmetry, each of these states is fully gapped with gap minimum centered above the normal state Fermi surface, in agreement with photoemission data of Y. Zhang et al. The analysis provides additional insights into the possible pairing mechanism for each of these states, highlighting the detrimental role of the renormalized repulsive intra-orbital Hubbard $U$ and inter-orbital $U'$ , and the beneficial role of the pair hopping $J'$ and the Hunds $J$ terms, as well as the spin-orbit coupling in the effective low-energy Hamiltonian.