Electronic structure of mixed-valence semiconductors in the LSDA+ U approximation. I. Sm monochalcogenides

The electronic structure and optical spectra of Sm monochalcogenides are investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with the so-called LSDA + U approach. In contrast to LSDA, where the stable solution in SmS is a metal, the LSDA + U gave an insulating ground state. The energy band structure of samarium monochalcogenides describes well their measured x-ray photo-emission spectra (XPS) as well as their optical spectra. The electronic structure of SmS high-pressure golden phase calculated in the LSDA+U approximation is characterized by five fully occupied 4f levels situated around 6 eV below the Fermi level and a sixth 4f level partly occupied due to pinning at the Fermi level. The occupation number is equal to 0.45 (valence 2.55+).