Raman response in the nematic phase of FeSe

TL;DR

This paper explains the rapid decrease of the $B_{1g}$ Raman response in FeSe's nematic phase as a consequence of orbital content changes and vertex corrections, aligning with experimental observations.

Contribution

It introduces a theoretical explanation for the Raman response behavior in FeSe's nematic phase, emphasizing the role of orbital content and vertex corrections.

Findings

Raman response drops below the nematic transition due to orbital changes.

Vertex corrections reduce the Raman response, mimicking a gap-like behavior.

The theory aligns with experimental Raman data in FeSe.

Abstract

Raman experiments on bulk FeSe revealed that the low-frequency part of $B_{1g}$ Raman response $R_{B_{1g}}$, which probes nematic fluctuations, rapidly decreases below the nematic transition at $T_n \sim 85$K. Such behavior is usually associated with the gap opening and at a first glance is inconsistent with the fact that FeSe remains a metal below $T_n$, with sizable hole and electron pockets. We argue that the drop of $R_{B_{1g}}$ in a nematic metal comes about because the nematic order drastically changes the orbital content of the pockets and makes them nearly mono-orbital. In this situation $B_{1g}$ Raman response gets reduced by the same vertex corrections that enforce charge conservation. The reduction holds at low frequencies and gives rise to gap-like behavior of $R_{B_{1g}}$, in full agreement with the experimental data.