Nature of excitons and their ligand-mediated delocalization in nickel dihalide charge-transfer insulators
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2024
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| author | 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 |
| author_facet | 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 |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2404_10818 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Nature of excitons and their ligand-mediated delocalization in nickel dihalide charge-transfer insulators 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 Strongly Correlated Electrons Materials Science 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. |
| title | Nature of excitons and their ligand-mediated delocalization in nickel dihalide charge-transfer insulators |
| topic | Strongly Correlated Electrons Materials Science |
| url | https://arxiv.org/abs/2404.10818 |