二维反铁磁(NixFe1-x)2P2S6单晶及其薄片中的共振增强多声子散射与局域轨道-声子耦合

中文摘要 AI 辅助

轨道自由度在塑造二维范德华磁性体系的物理性质中起着关键作用，强烈影响电子-声子耦合和中间态动力学。在本研究中，我们对二维范德华反铁磁系列(NixFe1-x)2P2S6的单晶进行了全面的温度和厚度依赖的拉曼研究。利用拉曼光谱，我们探索了局域轨道激发与晶格振动之间的相互作用，观察到高达四阶的高阶声子模式。我们发现这些高阶模式相对于低能一阶声子的强度比异常高且对温度依赖较弱。理论团簇计算和Franck-Condon模型表明，这些特征源于由局域Ni 3d8多重态激发介导的共振增强多声子散射。这些中间态的局域轨道占据与晶格坐标强耦合，为特定声子通道提供了选择性增强机制。值得注意的是，这些高阶特征在端元Fe2P2S6中缺失。这一差异反映了Ni2P2S6的电荷转移特性，其光学跃迁与局域多重态共振，与富铁类似物中的Mott-Hubbard绝缘体区间形成对比。我们的发现阐明了过渡金属硫代磷酸盐中高频拉曼散射的微观起源，并强调了局域轨道-声子相互作用在二维关联磁体中的核心作用。

英文摘要

Orbital degrees of freedom play a pivotal role in shaping the physical properties of two-dimensional (2D) van der Waals magnetic systems, strongly influencing electron-phonon coupling and intermediate-state dynamics. In this study, we present a comprehensive temperature and thickness-dependent Raman investigation of the single crystals of 2D van der Waals antiferromagnetic series (NixFe1-x)2P2S6. Utilizing Raman spectroscopy, we explore the interplay between local orbital excitations and lattice vibrations, observing higher-order phonon modes extending up to the fourth order. We observe an anomalously high and weakly temperature-dependent intensity ratio of these higher-order modes relative to low-energy first-order phonons. Theoretical cluster calculations and Franck-Condon modelling reveal that these features originate from resonantly enhanced multiphonon scattering mediated by localized intermediate Ni 3d8 multiplet excitations. The local orbital occupancy of these intermediate states couples strongly to lattice coordinates, providing a selective enhancement mechanism for specific phonon channels. Notably, these higher-order features are absent in the end-member Fe2P2S6. This distinction reflects the charge-transfer character of Ni2P2S6, which places optical transitions in resonance with local multiplet states, contrasted with the Mott-Hubbard insulator regime in Fe-rich analogues. Our findings clarify the microscopic origin of high-frequency Raman scattering in transition metal thiophosphates and underscore the central role of local orbital-phonon interactions in 2D correlated magnets.

发表机构

• Indian Institute of Technology Mandi（印度技术学院曼迪分校）
• Wrocław University of Science and Technology（弗罗茨瓦夫科技大学）
• Leibniz-Institute for Solid-state and Materials Research, IFW-Dresden（德累斯顿IFW莱布尼茨固体与材料研究所）
• Technische Universität Dresden（德累斯顿工业大学）
• International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所磁顶国际研究中心）
• University of Warsaw（华沙大学）

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