Collapse of critical nematic fluctuations in FeSe under pressure

TL;DR

This study investigates how electronic nematic fluctuations in FeSe change under pressure, revealing their collapse above 1.6 GPa and identifying two distinct nematic regimes linked to different physical mechanisms.

Contribution

It provides the first detailed Raman scattering analysis of nematic susceptibility evolution in FeSe under pressure, connecting fluctuation suppression to electronic and lattice changes.

Findings

Nematic fluctuations vanish above 1.6 GPa.

Collapse linked to suppression of low energy electronic excitations.

Identification of two nematic regimes: low-pressure Pomeranchuk instability and high-pressure magnetic orthorhombic distortion.

Abstract

We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature $T_{c}$ reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.6 GPa, indicating they play a marginal role in the four-fold enhancement of $T_{c}$ at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical and electronic structure calculations using local density approximation combined with dynamical mean field theory. Our results reveal two different regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave Pomeranchuk instability of the Fermi surface at low pressure and a magnetic driven orthorhombic distortion at higher pressure.