Phase transition of monolayer Pb/Ge(111): β-√3 × √3 R30°⇆1×1 at ∼180 °C

We study a reversible phase transition of 1 monolayer (ML) of Pb on a Ge(111) surface using a scanning tunneling microscope (STM) over a temperature range from below to above the transition (∼175 °C). Below the transition temperature, a periodic array of Pb trimers centered on ${\mathrm{H}}_3$ sites is resolved. The controversial high-temperature phase is actually a simple Pb-terminated 1×1 structure. Just above the transition temperature, we observe a strongly fluctuating structure. Between 1/3 and 1 ML of Pb coverage, the transition is found to start in the vicinity of the phase boundary with a 1/3-ML √3 phase. At 1 ML we also observe the transition beginning at the step edges on the upper terraces. A sharp increase in the transition temperature occurs near 1 ML. The transition temperature is also found to depend on the domain size. The role of surface-strain fields on the phase transformation is discussed.