# Regensburg 2019 – wissenschaftliches Programm

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# KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

## KFM 14: Postersession KFM

### KFM 14.18: Poster

### Mittwoch, 3. April 2019, 16:00–18:30, Poster C

**The K-edge of Hexagonal Boron Nitride Revisited: Effects of Electron-phonon Coupling on the Absorption Spectrum** — •Ferenc Karsai^{1}, Moritz Humer^{2}, Georg Kresse^{1,2}, Espen Flage-Larsen^{3}, and Peter Blaha^{4} — ^{1}VASP Software GmbH, Sensengasse 8, Vienna, Austria — ^{2}University of Vienna, Department of Physics, Sensengasse 8, Vienna, Austria — ^{3}SINTEF Materials and Chemistry, Oslo, Norway — ^{4}Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9/165-TC, Vienna, Austria

The theoretical X-ray absorption near-edge structure for the boron and nitrogen K-edge in hexagonal boron nitride is investigated in great detail employing density-functional theory calculations. Electron-hole interactions are described using the super-cell core-hole method and the Bethe-Salpeter equation. The calculations are carried out with two different codes, the VASP and the WIEN2k code employing the projector augmented wave and the full-potential linear augmented plane-wave method, respectively. Close agreement between spectra obtained from the two codes is found. However, the boron spectrum obtained in the absence of vibrational effects shows significant deviations from experiment. Particularly the calculated spectrum yields a single 2p sigma peak and hence totally fails to describe the experimentally observed double-peak structure. By the inclusion of electron-phonon interactions the theoretical 2p sigma peak observes a significant splitting. We incorporate these effects in our calculations fully parameter-free and ab initio using a one-shot sampling method and obtain excellent agreement with experiment.