Core and valence electron excitations in SrTiO₃ and MgO: a first-principles study including many-body effects

Using density functional theory calculations combined with many-body perturbation theory we investigate the optical and XAS spectra of two paradigmatic oxides, SrTiO₃ and MgO. For both systems taking into account quasiparticle (GW) and in particular excitonic effects (Bethe-Salpeter equation) is decisive to obtain good agreement with experiment. For the cubic phase of SrTiO₃ [1], the theoretical optical spectrum shows a prominent excitonic peak at 6.58 eV which involves interband transition between O-2p and Ti-e_g states. The main effect of the tetragonal distortion below 105 K is observed around this peak due to splitting of the e_g bands. The optical spectrum of MgO shows the best agreement with experiment using a hybrid exchange-correlation functional. Furthermore, the x-ray absorption spectra of the O and Mg K-edge are in good agreement with experiment. The analysis of the origin of the peaks in k-space indicates a strong hybridization of the respective unoccupied p and d-states, whereas the real space visualization of the exciton wavefunction illustrates its localization and bound nature.
[1] V. Begum, M. E. Gruner and R. Pentcheva, Phys. Rev. Materials 3, 065004 (2019).