Density of Phonon States in Superconducting FeSe as a Function of Temperature and Pressure

TL;DR

This study investigates how temperature and pressure affect the phonon density of states in superconducting FeSe, revealing significant phonon energy shifts under pressure but indicating that electron-phonon coupling alone cannot explain the increased superconducting temperature.

Contribution

It provides detailed measurements of phonon spectral changes in FeSe under varying conditions, highlighting the limited role of electron-phonon interactions in its superconductivity.

Findings

Phonon modes sharpen and shift to higher energies as temperature decreases.

Pressure significantly increases phonon energies, correlating with higher Tc.

Electron-phonon coupling alone does not account for Tc enhancement under pressure.

Abstract

The temperature and pressure dependence of the partial density of phonon states of iron atoms in superconducting Fe1.01Se was studied by 57Fe nuclear inelastic scattering (NIS). The high energy resolution allows for a detailed observation of spectral properties. A sharpening of the optical phonon modes and shift of all spectral features towards higher energies by ~4% with decreasing temperature from 296 K to 10 K was found. However, no detectable change at the tetragonal - orthorhombic phase transition around 100 K was observed. Application of a pressure of 6.7 GPa, connected with an increase of the superconducting temperature from 8 K to 34 K, results in an increase of the optical phonon mode energies at 296 K by ~12%, and an even more pronounced increase for the lowest-lying transversal acoustic mode. Despite these strong pressure-induced modifications of the phonon-DOS we conclude that the pronounced increase of Tc in Fe1.01Se with pressure cannot be described in the framework of classical electron-phonon coupling. This result suggests the importance of spin fluctuations to the observed superconductivity.