Optical investigation of the metal-insulator transition in FeSb<Subscript>2</Subscript>

We present a comprehensive optical study of the narrow gap FeSb<Subscript>2</Subscript> 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 σ<Subscript>1</Subscript>(ω) of the complex optical conductivity. With decreasing temperature below 80 K, we find a progressive depletion of σ<Subscript>1</Subscript>(ω) below E<Subscript>g</Subscript>∼300 cm<Superscript>-1</Superscript>, the semiconducting optical gap. The suppressed (Drude) spectral weight within the gap is transferred at energies ω>E<Subscript>g</Subscript> and also partially piles up over a continuum of excitations extending in the spectral range between zero and E<Subscript>g</Subscript>. Moreover, the interaction of one phonon mode with this continuum leads to an asymmetric phonon shape. Even though several analogies between FeSb<Subscript>2</Subscript> and FeSi were claimed and a Kondo-insulator scenario was also invoked for both systems, our data on FeSb<Subscript>2</Subscript> differ in several aspects from those of FeSi. The relevance of our findings with respect to the Kondo insulator description will be addressed. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006