快速电子向大尺寸球形纳米颗粒传递角动量的谱密度
Spectral density of angular momentum transfer from a swift electron to a large spherical nanoparticle
- Universidad Nacional Autónoma de México（墨西哥国立自治大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
提出一种高效多极收敛方法，计算快速电子向大球形纳米颗粒传递角动量的谱密度，揭示干涉主导及电/磁贡献差异，并发现金比铝传递更多角动量。
AI中文摘要：
扫描透射电子显微镜中的快速电子向纳米颗粒同时传递线动量和角动量，这是电子束驱动的纳米尺度操控（“电子镊子”）的基础。先前的理论要么依赖于小颗粒（偶极）近似，该近似仅在远低于实验相关尺寸时有效；要么依赖于频率积分的多极计算，这无法解析相互作用的谱结构。在此，我们提出一种完全推迟、因果、多极收敛的电动动力学方法，用于计算快速电子向孤立球形纳米颗粒的角动量传递。该方法基于闭合曲面麦克斯韦应力张量表述，其角度积分可解析地约化为一组小的、与材料和轨迹无关的连带勒让德函数不可约积分。这将双多极求和的成本从O(l_max^4)降低到O(l_max^3)，使得对于a=50 nm的纳米颗粒能够收敛到l_max=51，这几乎是该尺寸下先前最大计算阶数的四倍，并且对于光学复杂材料而言是前所未有的，同时成本降低了三到四个数量级。将该方法应用于铝和金纳米颗粒（最大a=50 nm），该方法解析了全频率域内传递的谱密度，表明其由电子与散射场之间的干涉决定，且几乎在每个频率上都占主导；电贡献比磁贡献高出两到三个数量级，尽管谱分辨揭示了频率积分传递中隐藏的磁项符号变化。在a=50 nm时，尽管金的响应更为复杂，但其传递的角动量显著多于铝，且比值随速度增加，从v=0.5c时的约2倍增长到v→c时的超过6倍（固定b=51 nm，从纳米颗粒中心算起）。
英文摘要：
Swift electrons in scanning transmission electron microscopy transfer both linear and angular momentum to nanoparticles, underlying electron-beam-driven nanoscale manipulation ("electron tweezers"). Prior theory relied either on the small-particle (dipolar) approximation, valid only well below experimentally relevant sizes, or on frequency-integrated multipolar calculations that leave the spectral structure of the interaction unresolved. Here we present a fully retarded, causal, multipole-converged electrodynamical methodology for the angular momentum transfer from a swift electron to an isolated spherical nanoparticle, based on a closed-surface Maxwell stress tensor formulation whose angular integrals reduce analytically to a small, material- and trajectory-independent set of irreducible integrals over associated Legendre functions. This lowers the cost of the double multipolar sum, enabling convergence up to l_max=51 for nanoparticles as large as a=50 nm, nearly four times the order of the largest previous calculation at this size and previously unreached for an optically complex, interband-dominated material. Applied to aluminum and gold nanoparticles up to a=50 nm, the method resolves the transfer spectral density across the full frequency domain, showing it is set by interference between the electron field and the field scattered by the nanoparticle, which dominates the scattered-scattered contribution at essentially every frequency; the electric contribution dominates at moderate speeds, but the magnetic contribution, of the same sign, grows steadily in relative weight with electron speed, from a few percent of the total at v = 0.5c to between a quarter and a half of it at v = 0.95c for trajectories passing close to the nanoparticle surface.
