Long-lived coherent states in photo-excited NiO

Charge excitations across electronic band gaps are a key ingredient for transport in optoelectronics and light harvesting applications. In contrast to conventional semiconductors, studies of above-band-gap photoexcitations in strongly correlated materials are still in their infancy.
We will present a combined experimental and theoretical study of the ultrafast dynamics in strongly correlated photo-excited NiO [1]. By combining time-resolved two-photon photoemission experiments and the non-equilibrium extension of dynamical mean-field theory we revealed an ultrafast electronic relaxation (<10 fs) and related photo-induced in-gap states connected with the Hund excitations. Remarkably, the weight of these in-gap states displays long-lived coherent THz oscillations up to 2 ps. The frequency of these oscillations corresponds to the strength of the antiferromagnetic superexchange interaction in NiO and their lifetime vanishes as the Neel 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.

[1] K. Gillmeister, D.Golez, C. Chiang, N. Bittner, P. Werner, Y. Pavlyukh, J. Berakdar, and W. Widdra, arXiv:1909.00828 (2019).