Density of phonon states in superconducting FeSe as a function of temperature and pressure

The temperature and pressure dependence of the partial density of phonon states (phonon-DOS) of iron atoms in superconducting Fe_1.01 Se was studied by ⁵⁷Fe nuclear inelastic scattering. 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 toward higher energies by ∼4% with decreasing temperature from 296 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 in the superconducting temperature from 8 to 34 K, results in an increase in 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 in T_c in Fe_1.01 Se 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.