Optical investigation of the metal-insulator transition in $FeS{b}_2$

Abstract

We present a comprehensive optical study of the narrow gap $FeS{b}_2$ 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 $\ddot{I}{ƒ}_1(\ddot{I}‰)$ of the complex optical conductivity. With decreasing temperature below 80 K, we find a progressive depletion of $\ddot{I}{ƒ}_1(\ddot{I}‰)$ below $E_g\sim 280$ cm${}^{-1}$, the semiconducting optical gap. The suppressed (Drude) spectral weight within the gap is transferred at energies $\ddot{I}‰>E_g$ 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 $FeS{b}_2$ and $FeSi$ were claimed and a Kondo-insulator scenario was also invoked for both systems, our data on $FeS{b}_2$ differ in several aspects from those of $FeSi$. The relevance of our findings with respect to the Kondo insulator description will be addressed.

17 pages, 5 figures