Measuring the ultrafast screening of $U$ in photo-excited charge-transfer insulators with time-resolved X-ray absorption spectroscopy

TL;DR

This paper investigates how time-resolved X-ray absorption spectroscopy can measure ultrafast changes in electron interactions, specifically the Hubbard U, in photo-excited charge-transfer insulators using advanced theoretical models.

Contribution

It demonstrates the effectiveness of XAS for probing dynamical screening effects on electron interactions and clarifies limitations in interpreting spectral shifts.

Findings

XAS can measure changes in electron interactions due to dynamical screening.

Screening affects both the position and lineshape of absorption lines.

Single-parameter U modifications may not fully capture spectral shifts.

Abstract

Recent seminal experiments have utilized time-resolved X-ray absorption spectroscopy (XAS) to investigate the ultrafast photo-induced renormalization of the electron interaction (''Hubbard $U$'') in Mott and charge transfer insulators. In this paper, we analyze the change of interactions due to dynamical screening as it is encoded in the XAS signal, using the non-equilibrium 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 photo-excited 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 $\Delta U$ extracted from the shift of the XAS lines does not necessarily describe the related shift of the the upper Hubbard band. We clarify these aspects using a minimal cluster model and the three-band Emery model for a charge transfer insulator.