Electronic structure of Sm monochalcogenides

The optical reflectivity of single crystals of semiconducting SmS, SmSe, and SmTe as well as of the metallic, high-pressure phase of SmS has been investigated in the photon energy range between 0.03 and 12 eV, for metallic SmS also between 4.2 and 300 K. The dielectric functions have been derived by means of a Kramers-Kronig analysis and been interpreted by interband and intraband transitions. In addition, semiconducting evaporated films of SmS have been prepared and the absorption in function of temperature (4.2-300 K) and pressure (0-1.5 kbar) has been investigated. It can be shown that the electronic structure of the semiconducting Sm monochalcognides can be obtained from the corresponding one of the Eu monochalcogenides by a simple uniform shift in the energy of the $4f$ electronic state. Valence fluctuations in single crystalline, metallic SmS are present to a much lower degree than assumed so far.