Combined first principles and many-body theory modelling of x-ray absorption spectra of bulk MgO and SrTiO₃

We present a comprehensive study of the x-ray absorption spectra (XAS) in two paradigmatic oxides − MgO and SrTiO₃, from first-principles calculations. The spectra are calculated by including the quasiparticle corrections with G₀W₀ (MgO) / within the independent particle approximation (SrTiO₃), followed by the excitonic effects by solving the Bethe-Salpeter Equation (BSE). Our results show that inclusion of the electron – core hole interaction with BSE is integral to describe the spectra accurately. The simulated XAS spectra for the O and Mg K-edge (MgO) [1], and O K-edge (SrTiO₃) [2] are in excellent agreement with experiment w.r.t. the spectral shape and peak positions. The theoretical Ti-L_2,3 edge is concurrent with experiment w.r.t. the energetic positions of the four-peak structure stemming from the crystal-field splitting due to the Ti octahedral coordination in SrTiO₃. We also analyze the origin of prominent peaks and identify the orbital character of the relevant contributions by projecting the e-h coupling coefficients from the BSE eigenvectors on the band structure. The real-space projection of the wave functions for the lowest energy exciton of the O K-edge shows a strong localization (MgO), whereas a two-dimensional spread in the x-y plane is observed for SrTiO₃.