Visualizing Nanoscopic Acoustic Mode Competition in van der Waals Ferroelectric

TL;DR

This study uses ultrafast electron microscopy and diffraction to visualize and analyze the nanoscopic acoustic phonon dynamics in a van der Waals ferroelectric, revealing anisotropic polarization-strain coupling and heterogeneity in energy dissipation.

Contribution

It provides the first spatiotemporal visualization of acoustic phonons in a van der Waals ferroelectric, uncovering mode-specific lifetimes and spatial heterogeneity in energy dissipation mechanisms.

Findings

Identification of three acoustic phonons: two transverse shear modes and one longitudinal breathing mode.

Transverse shear mode perpendicular to polar axis dominates, indicating anisotropic coupling.

Regions with single shear mode have longer acoustic lifetimes, highlighting phonon-phonon scattering as a decoherence source.

Abstract

Understanding how low-dimensional ferroelectrics respond to ultrafast excitation at nanoscales is essential for controlling energy flow and mechanical functionality in next-generation polar devices, yet the nanoscopic structural response to ultrafast depolarization remains unresolved, obscuring the microscopic pathways of acoustic decoherence and energy dissipation. Here, we spatiotemporally resolve lattice motion in the van der Waals ferroelectric NbOI2 using combined ultrafast electron microscopy and diffraction, revealing three acoustic phonons: two transverse shear modes and one longitudinal breathing mode. The transverse mode that shears the layers perpendicular to the in-plane polar axis dominates over that along the polar axis, reflecting anisotropic polarization-strain coupling. Real-space mapping uncovers spatially correlated heterogeneity in mode amplitudes and lifetimes. Regions dominated by a single shear mode exhibit significantly longer acoustic lifetimes than multimode regions, suggesting acoustic phonon-phonon scattering as a major source of decoherence. Our results provide a microscopic understanding of ultrafast depolarization-driven acoustic dynamics and spatially heterogeneous energy dissipation in van der Waals ferroelectrics.