Phase transitions are often thought of macroscopically, as arising from big changes in the structure of a material, such as when ice melts, or water turns to steam, but of course these changes must be linked to events that happen atom by atom. Now a team at the Universidad Autonoma de Madrid in Spain has managed not only to see the structure of two phases, atom by atom, but to film the transition between them.
The team used a special scanning-tunnelling electron microscope stabilized so that it can track the same set of atoms in a sample, even as the temperature of the sample changes. Looking at a film of lead deposited on germanium, the researchers could watch a small array of atoms as the film switched between a smooth phase at temperatures above 86 K and a corrugated one below. They also observed similar behaviour for lead on silicon, and obtained results that agree with theories that assume that point-like defects do not play critical roles in such phase transitions.
Further reading
I Brihuega et al. 2005 Phys. Rev. Lett. 94 046101.