Understanding the vibrational properties of materials is crucial for predicting their optical, thermal, and elastic behaviors. While traditional methods offer limited spatial and energy resolution, a new momentum-selective electron energy-loss spectroscopy (EELS) technique enables atomic-resolution imaging of frequency- and symmetry-dependent vibrational anisotropies. This groundbreaking approach directly visualizes vibrational anisotropy in individual phonon modes, revealing insights previously inaccessible.
This study utilized strontium titanate (STO) and barium titanate (BTO) as model perovskite systems. In STO, two distinct oxygen vibration types were observed: oblate thermal ellipsoids below 60 meV and prolate ones above 60 meV, indicating a frequency-dependent change in the anisotropy of atomic displacements. This variation is directly linked to the material's dielectric and thermal properties.
The non-centrosymmetric BTO exhibited a more complex behavior. The researchers detected subtle distortions in the oxygen octahedra through the modulation of q-selective signals between apical and equatorial oxygen sites near 55 meV. This observation is a consequence of the reduced crystal symmetry and may be related to ferroelectric polarization.
Theoretical modeling validated the experimental findings, demonstrating the reliability of the new EELS technique. The ability to visualize phonon eigenvectors at specific crystallographic sites with unprecedented spatial and energy resolution opens exciting avenues for research into a wide range of material properties, including dielectric, optical, thermal, and even superconducting behaviors. This method provides a powerful new tool for materials science, enabling deeper understanding and control of material properties at the atomic level.
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Originally published at: https://www.nature.com/articles/s41586-025-09511-z
