Nature of excitons and their ligand-mediated delocalization in nickel dihalide charge-transfer insulators

The fundamental optical excitations of correlated transition-metal compounds are typically identified with multielectronic transitions localized at the transition-metal site, such as \(dd\) transitions. In this vein, intense interest has surrounded the appearance of sharp, below band-gap optical tra...

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Bibliographic Details
Published in:arXiv.org 2024-04
Main Authors: Occhialini, Connor A, Tseng, Yi, Elnaggar, Hebatalla, Song, Qian, Blei, Mark, Tongay, Seth Ariel, Bisogni, Valentina, Frank M F de Groot, Pelliciari, Jonathan, Comin, Riccardo
Format: Article
Language:English
Subjects:
Charge transfer
Excitation spectra
Excitons
Fine structure
Ground state
Inelastic scattering
Insulators
Ligands
Magnetic structure
Magnets
Metal compounds
Nickel
Transition metal compounds
X-ray scattering
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Summary:The fundamental optical excitations of correlated transition-metal compounds are typically identified with multielectronic transitions localized at the transition-metal site, such as \(dd\) transitions. In this vein, intense interest has surrounded the appearance of sharp, below band-gap optical transitions, i.e. excitons, within the magnetic phase of correlated Ni\(^{2+}\) van der Waals magnets. The interplay of magnetic and charge-transfer insulating ground states in Ni\(^{2+}\) systems raises intriguing questions on the roles of long-range magnetic order and of metal-ligand charge transfer in the exciton nature, which inspired microscopic descriptions beyond typical \(dd\) excitations. Here we study the impact of charge-transfer and magnetic order on the excitation spectrum of the nickel dihalides (NiX\(_2\), X \(=\) Cl, Br, and I) using Ni-\(L_3\) resonant inelastic x-ray scattering (RIXS). In all compounds, we detect sharp excitations, analogous to the recently reported excitons, and assign them to spin-singlet multiplets of octahedrally-coordinated Ni\(^{2+}\) stabilized by intra-atomic Hund's exchange. Additionally, we demonstrate that these excitons are dispersive using momentum resolved RIXS. Our data evidence a ligand-mediated multiplet dispersion, which is tuned by the charge-transfer gap and independent of the presence of long-range magnetic order. This reveals the mechanisms governing non-local interactions of on-site \(dd\) excitations with the surrounding crystal/magnetic structure, in analogy to ground state superexchange. These measurements thus establish the roles of magnetic order, self-doped ligand holes, and intersite coupling mechanisms for the properties of \(dd\) excitations in charge-transfer insulators.
ISSN:2331-8422