Photoinduced charge transfer renormalization in NiO

Photodoped states in strongly correlated charge-transfer insulators are characterized by 𝑑−𝑑 and 𝑑−𝑝 interactions and the resulting intertwined dynamics of charge excitations and local multiplets. Here, we use femtosecond x-ray absorption spectroscopy in combination with dynamical mean-field theory to disentangle these contributions in NiO. Upon resonant optical excitation across the charge-transfer gap, the Ni 𝐿₃ and O 𝐾 absorption edges red-shift for >10 ps, which we associate with photoinduced changes in the screening environment. An additional signature below the Ni 𝐿₃ edge is identified for <1 ps, reflecting a transient nonthermal population of local many-body multiplets. We employ a nonthermal generalization of the multiplet ligand field theory and argue that this feature originates from 𝑑−𝑑 transitions. Overall, the photodoped state differs significantly from a chemically doped state. Our results demonstrate that x-ray spectroscopies are helpful in revealing excitation pathways in correlated materials.