Quantitative prediction of the dynamics of in-gap states in correlated materials as seen in pump-probe PES, XAS and RIXS experiments: a NiO case study

This year the Nobel Prize for physics was awarded for attosecond studies of ultrafast dynamics in matter. Attosecond pump-probe experiments allow one to study and steer quantum materials on their fundamental time-scales. For atoms and small molecules one can theoretically predict the electronic and vibrational dynamics induced by ultra-fast light pulses [PRL 128, 153001 (2022), PRA 108, 032816 (2023)]. In solids a theoretical understanding is much harder. The coupling to many continuous degrees of freedom can result into rapid loss of coherence. Quantitative predictions how coherently driven excitations decohere is highly non-trivial. Correlated Mott-Hubbard or charge transfer insulators can show a variety of long lived excitonic excitations within the optical gap. With attosecond pump-probe spectroscopy it is possible to investigate the propagation and decay of such excitations, as recently shown by two-photon photo-emission spectroscopy of NiO. These experiments show photo-induced, long-lived in-gap states with coherent oscillations [Nat. Commun. 11, 4095 (2020)]. In this talk we will show, using non-linear response theory, how to quantitatively predict the dynamics of in-gap states in correlated materials after an optical excitation. We will furthermore show how this dynamics can be probed with different pump-probe experiments including photo-emission spectroscopy, x-ray absorption spectroscopy and resonant inelastic x-ray scattering.