# Optical conductivity and vibrational spectra of the narrow-gap semiconductor FeGa$_3$

## TL;DR

This study investigates the optical and vibrational properties of FeGa₃, revealing a direct band gap of about 0.7 eV, impurity states influencing spectral weight, and minimal electronic correlation effects.

## Contribution

It provides experimental optical and vibrational spectra of FeGa₃ and compares them with DFT calculations, highlighting impurity states and modest electronic correlations.

## Key findings

- FeGa₃ has a direct band gap of ~0.7 eV.
- Spectral weight below the gap decreases with temperature.
- Vibrational spectra show little temperature dependence and match DFT predictions.

## Abstract

Intermetallic narrow-gap semiconductors have been intensively explored due to their large thermoelectric power at low temperatures and a possible role of strong electronic correlations in their unusual thermodynamic and transport properties. Here we study the optical spectra and vibrational properties of $\mathrm{FeGa_3}$ single crystal. The optical conductivity indicates that $\mathrm{FeGa_3}$ has a direct band gap of $\sim 0.7$\,eV, consistent with density functional theory (DFT) calculations. Most importantly, we find a substantial spectral weight also below 0.4~eV, which is the energy of the indirect (charge) gap found in resistivity measurements and ab initio calculations. We find that the spectral weight below the gap decreases with increasing temperature, which indicates that it originates from the impurity states and not from the electronic correlations. Interestingly, we did not find any signatures of the impurity states in vibrational spectra. The infrared and Raman vibrational lines are narrow and weakly temperature dependent. The vibrational frequencies are in excellent agreement with our DFT calculations, implying a modest role of electronic correlations. Narrow M\" ossbauer spectral lines also indicate high crystallinity of the sample.