Cumulant expansion of the exciton Green's function: A unified approach for many-body intrinsic, extrinsic, and interference effects in XAS*

Recently, a real-time cumulant approach has been successful in describing many-body effects in x-ray photo-emission and absorption spectra in a variety of systems.¹ While the results for XAS are promising, intrinsic and extrinsic effects are not treated on the same footing, with the intrinsic excitations calculated via real-time TDDFT, while extrinsic/interference effects are calculated in a frequency space approach. In addition, extrinsic and interference effects are based on a model of free electrons interacting with plasmons, most appropriate for the alkali metals. Here we present a unified approach which treats satellites in terms of the real-time density response to the sudden appearance of an exciton, which is in turn described by solutions of the Bethe-Salpeter equation at a specific energy. We apply the method to the K-edge XAS of SrTiO3 and analyze the energy dependence of extrinsic and interference effects.
¹Kas et al, Phys. Rev. B 94, 035156 (2016)
²Vinson et al, Phys. Rev. B 83, 115106 (2011)