Hexatic phase melting observed in real-time in ultra-thin materials

Researchers at the University of Vienna have, for the first time, directly observed the melting process of ultra-thin, two-dimensional (2D) materials at the atomic level. Using a single atomic-thick layer of silver iodide (AgI) sandwiched between graphene sheets and heated inside a scanning transmission electron microscope (STEM), they captured real-time video of melting. This setup, combined with AI neural networks to track individual atoms across thousands of frames, allowed unprecedented insight into the phase transitions of 2D materials, which differ fundamentally from bulk materials due to atomic movement restricted to a flat plane. Their observations confirmed the existence of the hexatic phase—a state theorized since the 1970s but never directly seen in naturally bonded materials—where atomic distances become irregular like a liquid, yet angular order remains as in a solid. Importantly, while the transition from solid to hexatic phase occurred gradually as expected, the subsequent transition from hexatic to liquid was abrupt rather than continuous,