Optical investigation of the metal-insulator transition in $FeSb_2$

TL;DR

This optical study of FeSb2 reveals a temperature-dependent depletion of optical conductivity below its gap, showing spectral weight transfer and phonon interactions, with findings differing from similar Kondo insulators like FeSi.

Contribution

The paper provides a detailed optical characterization of FeSb2's metal-insulator transition, highlighting unique spectral features and interactions not observed in related compounds.

Findings

Depletion of optical conductivity below 280 cm$^{-1}$ at low temperatures

Spectral weight transfer from the gap to higher energies

Asymmetric phonon line shape due to interaction with continuum

Abstract

We present a comprehensive optical study of the narrow gap $FeSb_2$ semiconductor. From the optical reflectivity, measured from the far infrared up to the ultraviolet spectral range, we extract the complete absorption spectrum, represented by the real part $\sigma_1(\omega)$ of the complex optical conductivity. With decreasing temperature below 80 K, we find a progressive depletion of $\sigma_1(\omega)$ below $E_g\sim 280$ cm$^{-1}$, the semiconducting optical gap. The suppressed (Drude) spectral weight within the gap is transferred at energies $\omega>E_g$ and also partially piles up over a continuum of excitations extending in the spectral range between zero and $E_g$. Moreover, the interaction of one phonon mode with this continuum leads to an asymmetric phonon shape. Even though several analogies between $FeSb_2$ and $FeSi$ were claimed and a Kondo-insulator scenario was also invoked for both systems, our data on $FeSb_2$ differ in several aspects from those of $FeSi$. The relevance of our findings with respect to the Kondo insulator description will be addressed.