A combined inelastic neutron scattering and \textit{ab initio} lattice dynamics study of FeSi

TL;DR

This study combines inelastic neutron scattering and ab initio calculations to analyze phonon behavior during the semiconductor-to-metal transition in FeSi, revealing widespread phonon softening not limited to high-symmetry points.

Contribution

It provides a detailed momentum-resolved analysis of phonon softening in FeSi across the crossover, supported by combined experimental and theoretical approaches.

Findings

Phonon softening occurs at multiple Brillouin zone points.

Lattice dynamical calculations accurately reproduce the softening.

The results inform the debate on the microscopic origins of the crossover.

Abstract

The phonon renormalization across the semiconductor-to-metal crossover in FeSi is investigated by inelastic neutron scattering combined with \textit{ab-initio} lattice dynamical calculations. A significant part of reciprocal space with a particular focus on the 110$-$001 scattering plane is mapped by the time-of-flight inelastic neutron scattering data taken below and above the crossover. Individual momentum values are investigated in more detail as a function of temperature. The data reveal that the anomalous phonon softening upon metallization is not exclusive to the high symmetry $R$ and $\Gamma$ points. Several other phonon modes around the $R$-point as well as the phonon modes at the $M$ and $X$ points of the Brillouin zone exhibit anomalous phonon softening with magnitudes comparable to that observed at the $R$-point. The momentum dependence of the phonon softening is reproduced by the lattice dynamical calculation based on the density functional perturbation theory. We discuss our findings with respect to the nature of the semiconductor-to-metal crossover in FeSi, for which different microscopic origins have been proposed, i.e., lattice thermal disorder and electronic correlation effects.