# Ultrafast Transient Absorption Spectroscopy of the Charge-Transfer Insulator NiO: Beyond the Dynamical Franz-Keldysh Effect

## TL;DR

This paper uses advanced spectroscopy and computational methods to observe how the Hubbard U parameter dynamically changes in NiO, revealing new effects beyond the traditional Franz-Keldysh effect and suggesting potential phase transitions.

## Contribution

It introduces a self-consistent time-dependent DFT+U approach to observe dynamical Hubbard U effects in NiO, highlighting a new band-gap renormalization mechanism.

## Key findings

- Dynamical Hubbard U modulates transient optical properties.
- Observation of a dynamical band-gap renormalization.
- Potential for light-induced phase transitions in NiO.

## Abstract

We demonstrate that a dynamical modification of the Hubbard U in the model charge-transfer insulator NiO can be observed with state-of-the-art time-resolved absorption spectroscopy. Using a self-consistent time-dependent density functional theory plus U computational framework, we show that the dynamical modulation of screening and Hubbard U significantly changes the transient optical spectroscopy. Whereas we find the well-known dynamical Franz-Keldysh effect when the U is frozen, we observe a dynamical band-gap renormalization for dynamical U. The renormalization of the optical gap is found to be smaller than the renormalization of U. This work opens up the possibility of driving a light-induced transition from a charge-transfer into a Mott insulator phase.