Collective electronic excitations, including plasmons, excitons, and intra- and interband transitions, play a central role in determining the dynamic screening, optical response, and energy transport properties of materials. Recent advances in momentum- and frequency-resolved spectroscopies, such as electron energy-loss spectroscopy (EELS) and inelastic x-ray scattering (IXS), together with progress in first-principles many-body perturbation theory (MBPT) calculations, now allow collective excitations to be mapped with considerable precision across the Brillouin zone. This topical review surveys current developments in the representation and interpretation of both experimental and theoretical dielectric-response spectra. Particular emphasis is placed on recent ways of representing spectral band structures (SBS) of the direct and inverse dielectric functions, such as analytical approaches based on multipole-Padé approximants in momentum and frequency (MPA($\q$)), which provide a combined band-like description of the dispersion of the main collective excitations. We discuss how features observed in metals, semiconductors, and low dimensional systems reflect the interplay between electronic structure, screening strength, and local-field effects, and how post-processing procedures can improve the quantitative comparison between experiment and theory. Finally, we provide perspectives on open challenges and potential developments in quantitative dielectric-function analyses.
A tunable time-resolved spontaneous Raman spectroscopy setup for probing ultrafast collective excitation and quasiparticle dynamics in quantum materials
Order and randomness in dopant distributions: exploring the thermodynamics of solid solutions from atomically resolved imaging
作为掺杂剂浓度的函数的结构-性质关系的探索通常基于固溶体的平均场论。然而，这些理论在半导体中很好地工作，在具有强相关性的材料中往往会失败，无论是在电子行为还是化学分离方面。在这些情况下，原子排列的细节通常不会被探索和分析。材料的生成物理和化学的知识可以解决这个问题，因为可以生成作为原子级结构的随机表示的缺陷配置库，或者可以导出介观热力学模型的参数。为了获得这些信息，以改进预测，我们使用原子分辨的显微图像，可视化系统内复杂的结构相关性，并将其转化为结构形成的统计力学模型的数据。鉴于材料加工历史的微观方面存在显著的不确定性，以及可用图像的数量有限，我们将模型优化技术与统计假设检验的原则相结合。我们证明了从一系列的原子分辨扫描透射电子显微镜图像的Mo $_x$ Re $_{1-x}$ S $_2$在不同比例的Mo/Re化学计量的数据上的方法，为此，我们提出了一个有效的相互作用模型，然后用于生成原子配置，并在一系列的浓度和形成温度进行可测试的预测。
Spontaneous Symmetry Breaking and Collective Higgs-Goldstone Dynamics in Solid-State Phononic Frequency Combs
Birla Institute of Technology and Science, Pilani - Dubai Campus（比拉理工学院，皮拉尼-迪拜校区）；Birla Institute of Technology and Science, Pilani - Pilani Campus（比拉理工学院，皮拉尼-皮拉尼校区）
Optically driven collective spin excitations and magnetization dynamics in the Néel-type skyrmion host GaV$_4$S$_8$
GaV $_4$ S $_8$是一种多铁性半导体，具有磁性摆线（Cyc）和Néel型Skyrmion晶格（SkL）相，具有广泛的热稳定性和磁稳定性。在这里，我们使用时间分辨的磁光克尔光谱和微磁模拟来证明在Cyc和SkL阶段由单轴各向异性的光致调制驱动的集体自旋激发的相干产生。我们的研究结果揭示了自旋动力学在各向异性材料托管skyrmions和铺平了一条新的途径，其磁秩序的光学控制。
