Theory of ultrafast screening of U in driven charge-transfer insulators: a time-resolved x-ray absorption study

Recent seminal experiments have utilized time-resolved x-ray absorption spectroscopy (XAS) to investigate the ultrafast photoinduced renormalization of the electron interaction (“Hubbard U”) in Mott and charge transfer insulators. In this paper, we theoretically analyze the change of interactions due to dynamical screening as it is encoded in the XAS signal, using the nonequilibrium GW+EDMFT formalism. Our study shows that XAS is well-suited for measuring this change, but two aspects should be kept in mind if the screening processes are not substantially faster than the valence electron dynamics: (i) Screening in a photoexcited system can affect both the position and the lineshape of the absorption lines. (ii) In general, the effect cannot be captured by the modification of a single interaction parameter. Specifically, an estimate for Δ⁢� extracted from the shift of the XAS lines does not necessarily describe the related shift of the upper Hubbard band. We clarify these aspects using a minimal cluster model and the three-band Emery model for a charge transfer insulator.