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

IF 11.6 1区 物理与天体物理 Q1 PHYSICS, MULTIDISCIPLINARY Physical Review X Pub Date : 2024-07-12 DOI:10.1103/physrevx.14.031007
Connor A. Occhialini, Yi Tseng, Hebatalla Elnaggar, Qian Song, Mark Blei, Seth Ariel Tongay, Valentina Bisogni, Frank M. F. de Groot, Jonathan Pelliciari, Riccardo Comin
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Abstract

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 Ni2+ van der Waals magnets. The interplay of magnetic and charge-transfer insulating ground states in Ni2+ 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 (NiX2, X=Cl, Br, and I) using Ni-L3 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 Ni2+ 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.

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二卤化镍电荷转移绝缘体中激子的性质及其配位体介导的脱ocalization
相关过渡金属化合物的基本光学激发通常与过渡金属位点局部的多电子转变(如 dd 转变)有关。因此,人们对相关 Ni2+ 范德瓦耳斯磁体磁相中出现的尖锐、低于带隙的光学转变(即激子)产生了浓厚的兴趣。Ni2+ 体系中磁性和电荷转移绝缘基态的相互作用引发了关于长程磁序和金属配体电荷转移在激子性质中的作用的有趣问题,这激发了对典型 dd 激发之外的微观描述。在这里,我们利用 Ni-L3 边缘共振非弹性 X 射线散射 (RIXS),研究了电荷转移和磁序对镍二卤化物(NiX2,X=Cl、Br 和 I)激发光谱的影响。在所有化合物中,我们都探测到了类似于最近报道的激子的尖锐激发,并将其归结为八面体配位 Ni2+ 的自旋小卫星多子,通过原子内 Hund's 交换实现稳定。此外,我们还利用动量分辨 RIXS 证明了这些激子具有色散性。我们的数据证明了配体介导的多重子色散,这种色散由电荷转移间隙调节,与长程磁序的存在无关。这揭示了现场 dd 激发与周围晶体或磁性结构的非局部相互作用机制,类似于基态超交换。因此,这些测量结果确立了磁序、自掺杂配体空穴和位间耦合机制在电荷转移绝缘体中 dd 激发特性中的作用。
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来源期刊
Physical Review X
Physical Review X PHYSICS, MULTIDISCIPLINARY-
CiteScore
24.60
自引率
1.60%
发文量
197
审稿时长
3 months
期刊介绍: Physical Review X (PRX) stands as an exclusively online, fully open-access journal, emphasizing innovation, quality, and enduring impact in the scientific content it disseminates. Devoted to showcasing a curated selection of papers from pure, applied, and interdisciplinary physics, PRX aims to feature work with the potential to shape current and future research while leaving a lasting and profound impact in their respective fields. Encompassing the entire spectrum of physics subject areas, PRX places a special focus on groundbreaking interdisciplinary research with broad-reaching influence.
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