Eur. Phys. J. B 54, 175-183 (2006)
https://doi.org/10.1140/epjb/e2006-00433-1

Optical investigation of the metal-insulator transition in FeSb₂

¹ Laboratorium für Festkörperphysik, ETH Zürich, 8093 Zürich, Switzerland
² Condensed Matter Physics, Brookhaven National Laboratory, Upton, NY, 11973, USA
³ Physics Department, Brown University, Providence, RI, 02912, USA

Corresponding author: ^a degiorgi@solid.phys.ethz.ch

Received: 10 July 2006
Revised: 25 August 2006
Published online: 8 December 2006

Abstract

We present a comprehensive optical study of the narrow gap FeSb₂ 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 σ₁(ω) of the complex optical conductivity. With decreasing temperature below 80 K, we find a progressive depletion of σ₁(ω) below E_g∼300 cm^-1, the semiconducting optical gap. The suppressed (Drude) spectral weight within the gap is transferred at energies ω>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 FeSb₂ and FeSi were claimed and a Kondo-insulator scenario was also invoked for both systems, our data on FeSb₂ differ in several aspects from those of FeSi. The relevance of our findings with respect to the Kondo insulator description will be addressed.