Low-energy electronic structure of intermediate-valence "golden" SmS

Reflectivity measurements in the far-infrared region at 9 K show that the Fermi level of the intermediate-valence high-pressure phase of SmS lies in a gap of about 7 meV in width. At low temperatures the reflectivity has a double peak around 20 meV which is associated with intra-$4f$ transitions. The optical constants are calculated from a Kramers-Kronig analysis of the reflectivity. An oscillator fit was made that separates $f-f$ from $f-d$ transitions. The interband density of states was estimated for the giant $f$ density of states to be about 1200 states per rydberg and cell compared with a density of about 70 $d$ states per rydberg and cell. For the first time a fine structure in the density of $f$ states has been observed.