Electronic structure and spectroscopic properties of thulium monochalcogenides

The electronic structure of the thulium monochalcogenides TmS, TmSe, and TmTe is studied with several theoretical approaches. The total energy is evaluated with the self-interaction corrected local-spin density approximation, whereby the Tm ions are described with either twelve or thirteen localized $f$ electrons with the remaining electrons forming bands. The comparisons of these two scenarios reveal the valency shift of the Tm ion through the series. The spectral functions of $\mathrm{Tm}X$ compounds are calculated including multiplet effects, and they are compared to experimental x-ray photoemission spectra. The basic tool is the Hubbard-I approximation in which the embedding of an isolated ${f}^{n}$ ion in a solid is performed by modifying the crystal Hamiltonian as obtained from the local-density approximation with the atomic self-energy of the ion. The parameters of the model are obtained from the self-consistent band structure calculation. The agreement with experiment is excellent, reproducing all significant multiplet structures.