Revisit of the Orbital-Fluctuation-Mediated Superconductivity in LiFeAs: Nontrivial Spin-Orbit Interaction Effects on the Bandstructure and Superconducting Gap Function

TL;DR

This study investigates the impact of spin-orbit interaction on the superconducting gap structure in LiFeAs, providing evidence that orbital fluctuations play a key role in its pairing mechanism, aligning with experimental observations.

Contribution

It introduces a multiorbital gap equation incorporating spin-orbit interaction and demonstrates the importance of inter-orbital interactions in explaining LiFeAs's gap structure.

Findings

Strong inter-orbital interactions are essential to reproduce the observed gap structure.

Spin-orbit interaction modifies the Fermi surface topology significantly.

Orbital-fluctuation pairing mechanism is supported by the gap structure analysis.

Abstract

Precise gap structure in LiFeAs (Tc = 18 K) given by ARPES studies offers us significant information to understand the pairing mechanism in iron-based superconductors. The most remarkable characteristics in LiFeAs gap structure would be that "the largest gap emerges on the tiny hole-pockets around Z point". This result had been naturally explained in terms of the orbital-fluctuation scenario (T. Saito et al., Phys. Rev. B 90, 035104 (2014)), whereas an opposite result is obtained by the spin-fluctuation scenario. In this paper, we study the gap structure in LiFeAs by taking the spin-orbit interaction (SOI) into account, motivated by the recent ARPES studies that revealed the significant SOI-induced modification of the Fermi surface topology. For this purpose, we construct the two possible tight-binding models with finite SOI by referring the bandstructures given by different ARPES groups. In addition, we extend the gap equation for multiorbital systems with finite SOI, and calculate the gap functions by applying the orbital-spin fluctuation theory. On the basis of both SOI-induced band structures, main characteristics of the gap structure in LiFeAs are naturally reproduced only in the presence of strong inter-orbital interactions between (xz/yz - xy) orbitals. Thus, the experimental gap structure in LiFeAs is a strong evidence for the orbital-fluctuation pairing mechanism.