Nature of Excitons and Their Ligand-Mediated Delocalization in Nickel Dihalide Charge-Transfer Insulators

The fundamental optical excitations of correlated transition-metal compoundsare typically identified with multielectronic transitions localized at thetransition-metal site, such as dd transitions. In this vein, intense interesthas surrounded the appearance of sharp, below band-gap optical transitions,i.e. excitons, within the magnetic phase of correlated Ni^2+ van der Waalsmagnets. The interplay of magnetic and charge-transfer insulating ground statesin Ni^2+ systems raises intriguing questions on the roles of long-rangemagnetic order and of metal-ligand charge transfer in the exciton nature, whichinspired microscopic descriptions beyond typical dd excitations. Here westudy the impact of charge-transfer and magnetic order on the excitationspectrum of the nickel dihalides (NiX_2, X = Cl, Br, and I) using Ni-L_3resonant inelastic x-ray scattering (RIXS). In all compounds, we detect sharpexcitations, analogous to the recently reported excitons, and assign them tospin-singlet multiplets of octahedrally-coordinated Ni^2+ stabilized byintra-atomic Hund's exchange. Additionally, we demonstrate that these excitonsare dispersive using momentum resolved RIXS. Our data evidence aligand-mediated multiplet dispersion, which is tuned by the charge-transfer gapand independent of the presence of long-range magnetic order. This reveals themechanisms governing non-local interactions of on-site dd excitations withthe surrounding crystal/magnetic structure, in analogy to ground statesuperexchange. These measurements thus establish the roles of magnetic order,self-doped ligand holes, and intersite coupling mechanisms for the propertiesof dd excitations in charge-transfer insulators.