Infrared spectroscopic study of phonons coupled to charge excitations in FeSi

TL;DR

This study uses FTIR spectroscopy to analyze phonons and charge excitations in FeSi, revealing weak electron-phonon coupling and the nature of optical phonons and their interaction with electronic states.

Contribution

It provides detailed insights into the phonon-electron interactions and the optical properties of FeSi, highlighting the weak coupling and the effective charge of ions.

Findings

Four optical phonons observed, three with Fano line shape

Effective charge of Fe and Si ions approximately 4e

Weak electron-phonon coupling (lambda ~ 0.1)

Abstract

From an investigation of the optical conductivity of FeSi single crystals using FTIR spectroscopy in the frequency range from 30 to 20000 wavenumbers we conclude that the transverse effective charge of the Fe and Si ions is approximately 4e. Of the five optical phonons which are allowed by symmetry we observe only four, three of which have a Fano line shape presumably resulting from an interaction of these modes with the electronic continuum. We show that the large oscillator strength of the phonons results from a relatively weak coupling (lambda of the order of 0.1) of the lattice degrees of freedom to an electronic resonance above the semiconductor gap, which is also responsible for the large electronic polarizability of the medium.