Nature of Excitons and Their Ligand-Mediated Delocalization in Nickel Dihalide Charge-Transfer Insulators,Physical Review X

当前位置： X-MOL 学术 › Phys. Rev. X › 论文详情

Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)

Nature of Excitons and Their Ligand-Mediated Delocalization in Nickel Dihalide Charge-Transfer Insulators
Physical Review X ( IF 11.6 ) Pub Date : 2024-07-12 , DOI: 10.1103/physrevx.14.031007
Connor A. Occhialini ₁ , Yi Tseng ₁ , Hebatalla Elnaggar ₂ , Qian Song ₁ , Mark Blei ₃ , Seth Ariel Tongay ₃ , Valentina Bisogni ₄ , Frank M. F. de Groot ₅ , Jonathan Pelliciari ₄ , Riccardo Comin ₁

Affiliation

The fundamental optical excitations of correlated transition-metal compounds are typically identified with multielectronic transitions localized at the transition-metal site, such as

d d

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

d d

excitations. Here we study the impact of charge transfer and magnetic order on the excitation spectrum of the nickel dihalides (

Ni X_{2}

X = 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

{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

d d

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