  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.
