Ultrafast coupled charge and spin dynamics in strongly correlated NiO

TL;DR

This study uncovers ultrafast charge and spin dynamics in NiO driven by Hund's physics, revealing sub-10 femtosecond relaxation, long-lived THz oscillations, and the interplay between local excitations and magnetic order.

Contribution

It combines experimental and numerical approaches to demonstrate Hund physics' role in ultrafast dynamics of strongly correlated NiO, a relatively unexplored area.

Findings

Ultrafast ($\, extless$ 10 fs) relaxation due to Hund excitations.

Long-lived THz oscillations up to 2 ps at low temperature.

Oscillation frequency linked to antiferromagnetic superexchange.

Abstract

Charge excitations across an electronic band gap play an important role in opto-electronics and light harvesting. In contrast to conventional semiconductors, studies of above-band-gap photoexcitations in strongly correlated materials are still in their infancy. Here we reveal the ultrafast dynamics controlled by Hund's physics in strongly correlated photo-excited NiO. By combining time-resolved two-photon photoemission experiments with state-of-the-art numerical calculations, an ultrafast ($\lesssim$ 10\,fs) relaxation due to Hund excitations and related photo-induced in-gap states are identified. Remarkably, the weight of these in-gap states displays long-lived coherent THz oscillations up to 2\,ps at low temperature. The frequency of these oscillations corresponds to the strength of the antiferromagnetic superexchange interaction in NiO and their lifetime vanishes as the N\'eel temperature is approached. Numerical simulations of a two-band $t$-$J$ model reveal that the THz oscillations originate from the interplay between local many-body excitations and long-range antiferromagnetic order.