Superconductivity in FeSe thin films driven by the interplay between nematic fluctuations and spin-orbit coupling

TL;DR

This paper explores how nematic fluctuations and spin-orbit coupling influence superconductivity in FeSe thin films, revealing that their interplay favors an s-wave pairing state consistent with experimental observations.

Contribution

It demonstrates that nematic fluctuations alone promote degenerate pairing states, but spin-orbit coupling or inversion symmetry-breaking lifts this degeneracy, stabilizing an s-wave superconducting state.

Findings

Nematic fluctuations alone favor degenerate s- and d-wave pairing.

Spin-orbit coupling lifts degeneracy, favoring s-wave pairing.

The resulting gap anisotropy matches experimental data.

Abstract

The origin of the high-temperature superconducting state observed in FeSe thin films, whose phase diagram displays no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both $s$-wave and $d$-wave symmetries are equally favored, and $T_{c}$ is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry-breaking at the film interface lifts this harmful degeneracy and selects the $s$-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe and intercalated Li$_{1-x}$(OH)$_{x}$FeSe.