Real-Space Visualization of Frequency-Dependent Anisotropy of Atomic Vibrations

TL;DR

This paper presents a new electron energy-loss spectroscopy method to visualize atomic vibrational anisotropy in materials with high spatial and energy resolution, revealing detailed phonon behaviors.

Contribution

It introduces a novel momentum-selective vibrational spectroscopy technique using dark-field monochromated EELS to map phonon polarization vectors at specific atomic sites.

Findings

Successfully distinguished oxygen atom vibrational anisotropies in strontium titanate.

Demonstrated the ability to visualize phonon eigenvectors at specific crystalline sites.

Revealed frequency-dependent anisotropic vibrational behaviors.

Abstract

The underlying dielectric properties of materials, intertwined with intriguing phenomena such as topological polariton modes and anisotropic thermal conductivities, stem from the anisotropy in atomic vibrations. Conventionally, X-ray diffraction techniques have been employed to estimate thermal ellipsoids of distinct elements, albeit lacking the desired spatial and energy resolutions. Here we introduce a novel approach utilizing the dark-field monochromated electron energy-loss spectroscopy for momentum-selective vibrational spectroscopy, enabling the cartographic delineation of variations of phonon polarization vectors. By applying this technique to centrosymmetric cubic-phase strontium titanate, we successfully discern two types of oxygen atoms exhibiting contrasting vibrational anisotropies below and above 60 meV due to their frequency-linked thermal ellipsoids. This method establishes a new pathway to visualize phonon eigenvectors at specific crystalline sites for diverse elements, thus delving into uncharted realms of dielectric, optical, and thermal property investigations with unprecedented spatial resolutions.