Regensburg 2007 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 11: Poster Session

DF 11.22: Poster

Thursday, March 29, 2007, 14:30–18:00, Poster C

Optical properties of bulk CaF₂ and its F center — •Yuchen Ma and Michael Rohlfing — Fachbereich Physik, Universität Osnabrück, D-49069 Osnabrück, Germany

We present the quasiparticle band structures and optical excitation spectra of bulk CaF₂ and of the F center in CaF₂ using first-principle methods. The quasiparticle band structures are evaluated in Hedin’s GW approximation. Thereafter, the electron-hole interaction is calculated and the Bethe-Salpeter equation is solved, yielding the optical absorption spectra. The calculated quasiparticle band gap of bulk CaF₂ is 11.5 eV, which is in agreement with experiment (11.8 eV). The calculated optical absorption spectrum and reflectivity spectrum of bulk CaF₂, which consist of an exciton peak at 10.7 eV and several resonant-exciton peaks between 12 eV and 16 eV, are in good agreement with experiment. The first exciton peak corresponds to the excitation from the occupied F 2p orbitals to the s orbitals at the Γ-point of the lowest unoccupied conduction band.

One of the most prominent point defects of CaF₂ is given by the F center. The F center is characterized by a hole level which we find 6.6 eV above the top of the valence band. An exciton peak is observed at 3.5 eV for F center, which corresponds to the F center optical absorption peak (3.3 eV) observed in experiments. The excited electron is localized at the Ca atoms and F atoms surrounding the vacancy, exhibiting d-orbital character and p-orbital character, respectively. The hole occupies the s orbital centered at the vacancy and the p orbitals centered at the surrounding Ca atoms and F atoms.