# Identifying the dominant pairing interaction in high-$T_c$ FeSe superconductors through Leggett modes

## TL;DR

This paper proposes using Leggett modes observed via Raman scattering to identify the dominant pairing mechanism in high-$T_c$ FeSe superconductors, helping to clarify the origin of their enhanced transition temperatures.

## Contribution

It introduces a method to distinguish the main pairing interaction in FeSe superconductors by analyzing Leggett modes in a two-band model.

## Key findings

- Leggett mode can be detected in Raman $B_{1g}$ channel.
- Different fluctuations contribute to multiband interactions.
- Leggett mode presence indicates the dominant pairing mechanism.

## Abstract

Heavily electron-doped and single-layer FeSe superconduct at much higher temperatures than bulk FeSe. There have been a number of proposals attempting to explain the origin of the enhanced transition temperature, including the proximity to magnetic, nematic and antiferro-orbital critical points, as well as possible strong interfacial phonon coupling in the case of single-layer FeSe. In this paper, we examine the effect of the various mechanisms in an effective two-band model. Within our model, the fluctuations associated with these instabilities contribute to different parts of the effective multiband interactions. We propose to use the collective phase fluctuation between the bands--the Leggett mode--as a tool to identify the dominant effective pairing interaction in these systems. The Leggett mode can be resolved by means of optical probes such as electronic Raman scattering. We point out that the Leggett mode in these systems, if present, shall manifest in the Raman $B_{1g}$ channel.