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

Journal Article · 12 July 2024 · Physical Review. X

DOI: https://doi.org/10.1103/PhysRevX.14.031007 · OSTI ID:2397347

Occhialini, Connor A. ; Tseng, Yi ; Elnaggar, Hebatalla ; Song, Qian; Blei, Mark; Tongay, Seth Ariel; Bisogni, Valentina; de Groot, Frank M. F.; Pelliciari, Jonathan; Comin, Riccardo

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 ${\mathrm{Ni}}^{2+}$ van der Waals magnets. The interplay of magnetic and charge-transfer insulating ground states in ${\mathrm{Ni}}^{2+}$ 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 ( $\mathrm{Ni}{X}_2$ , $X=\mathrm{Cl}$ , Br, and I) using Ni- $L_3$ edge 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 ${\mathrm{Ni}}^{2+}$ 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 nonlocal interactions of on-site $dd$ excitations with the surrounding crystal or 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. Published by the American Physical Society 2024

• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0019126; SC0012704
• OSTI ID: 2397347
• Journal Information: Physical Review. X, Journal Name: Physical Review. X Journal Issue: 3 Vol. 14; ISSN PRXHAE; ISSN 2160-3308
• Publisher: American Physical SocietyCopyright Statement
• Country of Publication: United States
• Language: English