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 (approximately 175-degrees-C). The cont roversial high temperature phase is actually a simple Pb-terminated 1 x 1 structure. Below the transition temperature, Pb atoms are displace d from their T1 positions and form trimers with square-to 3 x square-r oot 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