Equipment Highlight: Atom-by-Atom Visualization of Moiré Phasons

Two‑dimensional (2D) materials with small twist angles form moiré superlattices whose unusual electronic and mechanical behaviors are governed by extremely low‑energy vibrational modes called phasons. These modes have been predicted to influence charge transport, thermal conductivity, and exciton motion, yet they had never been directly observed. Understanding these ultrasoft vibrations is essential for controlling the emergent properties of twisted 2D heterostructures and designing next‑generation quantum and electronic materials.

Enabled by an electron microscope pixel array detector in the Illinois MRSEC shared facilities, the team developed an ultrahigh‑resolution electron ptychography method to measure thermal vibrations atom‑by‑atom in twisted WSe₂ bilayer. By fitting the shape of atomic columns, we directly visualized the predicted signatures of moiré phasons, including enhanced and anisotropic vibrations at soliton domain walls and AA‑stacked regions, consistent with molecular dynamics and lattice‑dynamics calculations. This work provides the first images of moiré phasons and establishes a powerful new method for probing thermal vibrations at the atomic scale.