2025-09-17 カリフォルニア大学校アーバイン校(UCI)
Atomic vibrations, known as phonons, in crystalline materials propagate in a nonuniform, or anisotropic, manner, and this phenomenon governs the substances’ abilities to serve as building blocks for a wide variety of computing and energy technologies. UC Irvine researchers and international collaborators have invented a microscopy technique that helps them detect and image anisotropic atomic vibrations, a breakthrough that can lead to new advancements in high technology. Xiaoqing Pan / UC Irvine
＜関連情報＞
- https://news.uci.edu/2025/09/17/researchers-at-uc-irvine-are-first-to-image-directional-atomic-vibrations/
- https://www.nature.com/articles/s41586-025-09511-z
周波数依存フォノン異方性の原子スケールイメージング Atomic-scale imaging of frequency-dependent phonon anisotropy
Xingxu Yan,Paul M. Zeiger,Yifeng Huang,Haoying Sun,Jie Li,Chaitanya A. Gadre,Hongbin Yang,Ri He,Toshihiro Aoki,Zhicheng Zhong,Yuefeng Nie,Ruqian Wu,Ján Rusz & Xiaoqing Pan
Nature Published:17 September 2025
DOI:https://doi.org/10.1038/s41586-025-09511-z
Abstract
Directly visualizing vibrational anisotropy in individual phonon modes is essential for understanding a wide range of intriguing optical, thermal and elastic phenomena in materials1,2,3,4,5. Although conventional optical and diffraction techniques have been used to estimate vibrational anisotropies, they fall short in achieving the spatial and energy resolution necessary to provide detailed information4,5,6,7. Here, we introduce a new form of momentum-selective electron energy-loss spectroscopy, which enables the element-resolved imaging of frequency- and symmetry-dependent vibrational anisotropies with atomic resolution. Vibrational anisotropies manifest in different norms of orthogonal atomic displacements, known as thermal ellipsoids. Using the centrosymmetric strontium titanate as a model system, we observed two distinct types of oxygen vibrations with contrasting anisotropies: oblate thermal ellipsoids below 60 meV and prolate ones above 60 meV. In non-centrosymmetric barium titanate, our approach can detect subtle distortions of the oxygen octahedra by observing the unexpected modulation of q-selective signals between apical and equatorial oxygen sites near 55 meV, which originates from reduced crystal symmetry and may also be linked to ferroelectric polarization. These observations are quantitatively supported by theoretical modelling, which demonstrates the reliability of our approach. The measured frequency-dependent vibrational anisotropies shed new light on the dielectric and thermal behaviours governed by acoustic and optical phonons. The ability to visualize phonon eigenvectors at specific crystallographic sites with unprecedented spatial and energy resolution opens new avenues for exploring dielectric, optical, thermal and superconducting properties.
