AVS 51st International Symposium
    Surface Science Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA1
Real-Time Microscopy of Second Order Phase Transitions on Pb/Ge(111)

Tuesday, November 16, 2004, 1:20 pm, Room 210C

Session: Welch Award Symposium: Nucleation and Growth
Presenter: Y. Sato, University of California, Davis
Authors: Y. Sato, University of California, Davis
S. Chiang, University of California, Davis
Correspondent: Click to Email
Using the Low Energy Electron Microscope (LEEM), we have studied phase transitions that occur on the Pb/Ge(111) surface. We have observed a very unusual second order phase transition with strong fluctuations at coverages above 1ML. The Pb layer forms two stable (@sr@3x@sr@3)R30° phases on the surface, a low-density @alpha@ phase and a dense @beta@ phase. It is well known that the critical temperature of the phase transition from @beta@ to (1x1) depends strongly on the coverage. The transition occurs at 180C for coverage below 1ML and at 270C for coverages above 1.33ML. LEEM data clearly show this difference in the critical temperature derives from the different mechanisms of the transformation. For coverages between 0.33ML and 1ML, where the @alpha@ phase coexists with the @beta@ phase below the critical temperature and with the (1x1) phase above it, the reversible phase transformation from @beta@ to (1x1) shows typical second order phase transition behavior, i.e., the intensity of the @beta@ phase gradually changes with temperature. On the other hand, for coverages above 1 ML, where the surface is either the low temperature @beta@ phase or the higher temperature (1x1) phase, the system shows an unusual second order phase transition. As the sample is heated through the critical temperature, the (1x1) phase appears as small domains over the surface. The domains appear and disappear until they are stabilized in the new phase. The transition is reversible, and similar behavior is observed upon cooling. The frequency analysis of the critical fluctuations will be presented, and the different mechanisms for both of the second order phase transitions mentioned above will be discussed.