Ultrafast modification of the electronic structure of a correlated insulator

TL;DR

This study investigates the ultrafast electronic response of the correlated insulator NiO to strong optical fields, revealing unexpected stability of Ni 3d-orbitals and transient charge transfer phenomena, with implications for ultrafast device applications.

Contribution

The paper demonstrates that strong off-resonant optical fields do not alter Ni 3d-orbitals as predicted, and uncovers transient charge transfer in NiO, supported by first-principles theory.

Findings

No detectable change in Ni 3d-orbitals under strong fields

Observation of transient directional charge transfer

Highlights importance of retardation effects in electronic screening

Abstract

A non-trivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the timescales and energies involved in using quantum effects for possible applications. We use element-specific transient x-ray absorption spectroscopy and high-harmonic generation to measure the response to ultrashort off-resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V/{\AA} leads to no detectable changes on the correlated Ni 3d-orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first-principles theory. Our results highlight the importance of retardation effects in electronic screening, and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operation.