Visualizing nanoscopic acoustic mode competition in a van der Waals ferroelectric.

In the van der Waals ferroelectric NbOI2, ultrafast electron microscopy and diffraction resolve three acoustic phonons, with the transverse shear mode perpendicular to the in-plane polar axis dominating.

• Why it matters: Nanoscopic lattice responses to ultrafast depolarization are unresolved, limiting understanding of microscopic acoustic decoherence and energy dissipation pathways in polar devices.
• What they did: The team spatiotemporally mapped lattice motion in NbOI2 after ultrafast excitation using combined ultrafast electron microscopy and diffraction, extracting mode amplitudes and lifetimes across space.
• The result: Real-space mapping shows spatially correlated heterogeneity: regions dominated by a single shear mode have significantly longer acoustic lifetimes than multimode regions, suggesting phonon-phonon scattering as a major decoherence source.

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