AI 中文总结
研究长波长极限下激子色散，利用散焦工程动量分辨电子能量损失谱，以hBN为平台，解析层依赖激子色散，量化相关参数，研究其对环境响应，揭示低维激子物理，为调控激子特性提供指导并建立实验途径。
AI 中文摘要
激子色散对光电子学和量子材料至关重要，二维系统中长波长极限下预计有非解析激子色散，但缺乏其维度演化的直接定量表征。本文利用扫描透射电子显微镜中散焦工程动量分辨电子能量损失谱，以自由站立的hBN为平台，解析了层依赖的激子色散，量化了其特征交叉动量和群速度。随着厚度增加，非解析线性色散区域逐渐压缩。还研究了单层hBN激子能带结构对周围环境的响应。这些发现揭示了低维激子的基本物理，为调控层状量子材料中的激子输运等提供指导，并建立了探索低维激子物理的有力实验途径。
英文摘要
Exciton dispersion, which governs the propagation, scattering and radiative decay of electron-hole pairs, is essential to optoelectronics and quantum materials. In two-dimensional systems, weakened dielectric screening and long-range electron-hole exchange are predicted to induce nonanalytic exciton dispersion in the long-wavelength limit. However, direct quantitative characterization of its dimensional evolution remains lacking, especially in the ultralow-q regime (q < 0.02 $Å^{-1}$). Here we employ defocus-engineered momentum-resolved electron energy-loss spectroscopy in scanning transmission electron microscopy, achieving an ultrahigh momentum resolution of 0.0002 $Å^{-1}$. Using freestanding hBN as a prototypical platform, we resolve layer-dependent exciton dispersion and quantify its characteristic crossover momentum and group velocity in the long-wavelength limit. With increasing thickness, the nonanalytic linear-dispersion regime is progressively compressed, manifested by a reduction in characteristic crossover momentum q_c from $1.82 \times 10^{-1} Å^{-1}$ in the monolayer to $3.0 \times 10^{-1} Å^{-1}$ in 25 layers. Meanwhile, the low-q group velocity increases from $2.0 \times 10^{-3} c$ to $2.9 \times 10^{-2} c$, before the dispersion ultimately approaches the bulk-like parabolic limit. We further examine how the exciton band structure of monolayer hBN responds to its surrounding environment, including temperature, adjacent graphene layers, and interlayer twist in BN/graphene heterostructures. These findings uncover the fundamental physics of low-dimensional excitons, deliver valuable guidance for modulating exciton transport, diffusion and quasiparticle coupling in layered quantum materials, and establish a powerful experimental route to explore low-dimensional exciton physics.
