Strong electron correlations in FeSb2: An optical investigation and comparison with RuSb2

TL;DR

This study investigates the optical properties of FeSb2, revealing strong electron correlations and multiple charge gaps, and compares these findings with the structurally similar RuSb2, highlighting the role of electron-electron interactions.

Contribution

It provides the first detailed optical analysis of FeSb2 showing evidence of strong electron correlations and identifies multiple charge gaps, advancing understanding of its electronic structure.

Findings

FeSb2 exhibits a direct gap of 130 meV and indirect gaps at 6 and 31 meV.

Strong electron-electron correlations influence the charge gap formation.

Spectral weight redistribution indicates many-body effects beyond simple band theory.

Abstract

We report investigations of the optical properties of the narrow gap semiconductor FeSb2 in comparison with the structural homolog RuSb2. In the infrared region the latter shows insulating behavior in whole investigated temperature range (10 - 300 K) whereas the optical reflectivity of FeSb2 shows typical semiconductor behavior upon decreasing the temperature. The conduction electron contribution to the reflectivity is suppressed and the opening of a direct and an indirect charge excitation gap in the far-infrared energy region is observed. Those gap openings are characterized by a redistribution of spectral weight of the optical conductivity in an energy region much larger than the gap energies indicating that strong electron-electron correlations are involved in the formation of the charge gap. Calculations of the optical conductivity from the band structure also provided evidence for the presence of strong electronic correlations. Analyzing the spectra with a fundamental absorption across the gap of parabolic bands yields a direct gap at 130 meV and two indirect gaps at 6 and 31 meV. The strong reduction in the free-carrier concentration at low energies and low temperatures is also reflected in a change in the asymmetry of the phonon absorption which indicates a change in the phonon-conduction-electron interaction.