# Pump-probe Auger-electron spectroscopy of Mott insulators

## TL;DR

This paper explores how pump-probe Auger-electron spectroscopy reveals the dynamics of two-hole bound states in Mott insulators, highlighting the method's sensitivity to electronic correlations and screening effects.

## Contribution

It provides a theoretical analysis of nonequilibrium Auger spectra in Mott insulators, emphasizing the role of dynamical screening and orbital expansion effects.

## Key findings

- Spectra of photo-doped systems resemble higher filling equilibrium spectra.
- Bound two-hole states are highly stable over femtosecond timescales.
- Oscillations in satellite barycenter reflect dynamical Coulomb screening.

## Abstract

In high-resolution core-valence-valence (CVV) Auger electron spectroscopy from the surface of a solid at thermal equilibrium, the main correlation satellite, visible in the case of strong valence-electron correlations, corresponds to a bound state of the two holes in the final state of the CVV Auger process. We discuss the physical significance of this satellite in nonequilibrium pump-probe Auger spectroscopy by numerical analysis of a single-band Hubbard-type model system including core states and a continuum of high-energy scattering states. It turns out that the spectrum of the photo-doped system, due to the increased double occupancy, shares features with the equilibrium spectrum at higher fillings. The pumping of doublons can be watched when working with overlapping pulses at short $\Delta t$. For larger pump-probe delays $\Delta t$ and on the typical femtosecond time scale for electronic relaxation processes, spectra are hardly $\Delta t$-dependent, reflecting the high stability of bound two-hole states for strong Hubbard-$U$. We argue that taking into account the spatial expansion of single-particle orbitals when these are doubly occupied, as described by the dynamical Hubbard model, produces an oscillation of the barycenter of the satellite as a function of $\Delta t$. Pump-probe Auger-electron spectroscopy is thus highly sensitive to dynamical screening of the Coulomb interaction.