Tunneling microscope observation of a structural surface phase transition: Structure, fluctuations, and local effects

We study a reversible phase transition of one monolayer of Pb on a Ge(111) surface with a tunneling microscope over a temperature range from below to above the transition (∼175 °C). The controversial high temperature phase is actually a simple Pb-terminated 1×1 structure. Below the transition temperature, Pb atoms are displaced from their ${\mathit{T}}_1$ positions and form trimers with √3 × √3 periodicity. A fluctuating structure with patches of trimers can be resolved slightly above the transition temperature. The important effects of boundaries and domain size on the phase transformation are discussed.