Delocalization of Ubiquitous, Ultra-Sharp dd Excitations in Ni²⁺-based van der Waals Magnets Probed with Resonant Inelastic X-ray Scattering

Excitons are fundamental quasiparticles in the study of light-matter coupling of particular interest in van der Waals (vdW) semiconductors. Significant efforts have been given towards the study of novel excitations in magnetic vdW systems hosting Ni²⁺ions. Sharp peaks in optical experiments have been associated to spin-forbidden dd excitations, with a suggested excitonic character related to long-range magnetic order that is mediated by self-doped ligand holes. Significant evidence has been provided by resonant inelastic X-ray scattering (RIXS) experiments; however, this interpretation raises questions regarding the proposed excitonic delocalization mechanism and the true connection to long-range magnetism. Here, we investigate the electronic states of Ni²⁺ vdW magnets with Ni L₃ edge high-resolution RIXS, focusing on the nickel dihalides NiX₂ (X=Cl,Br,I). Variation of the halogen corresponds to tuning the covalency and the spin/orbital ground states, allowing us to assess the role of the ligand holes, electronic configuration and magnetism. We find near resolution-limited dd excitations of both ¹A_1g and ¹E_g character are ubiquitous in the nickel dihalides and likely a general feature of Ni²⁺ systems. The role of ligand holes can be effectively mapped to a screening of the intra-atomic Hund’s coupling within a single-ion model. We further reveal characteristic temperature and, particularly, momentum dependencies of these excitations that are qualitatively similar across the dihalide series despite disparate magnetic ground states/transition temperatures. From these observations, we conclude that these excitations are independent of magnetic order and are localized dd transitions, likely coupled to distinct bosonic excitations, such as phonons.