CRITICAL SCATTERING AT THE ORDER-DISORDER PHASE-TRANSITION OF SI(111)-ROOT-3X-ROOT-3R30-DEGREES-AU SURFACE - A PHASE-TRANSITION WITH PARTICLE EXCHANGE

The Si(111)-root 3 x root 3-Au phase, which coexists with three-dimens ional islands of excess Au at coverages above 1 monolayer and at tempe ratures above 700 K, is shown to undergo a temperature driven order-di sorder phase transition at 1057 K. The islands act as a reservoir of A u atoms and adjust the chemical potential for the two-dimensional laye r during the transition. Due to this particle exchange, the phase tran sition within the root 3 x root 3R30 degrees-Au has been observed for a chemisorbed layer which is not under the usual constraint of constan t coverage, but controlled by the chemical potential. We quantitativel y analyzed the critical scattering and experimentally determined its c ritical exponents beta, gamma, and nu by high-resolution low-energy el ectron diffraction. Their values are in good agreement with the expect ed values of the three-state Potts model, which shows that the transit ion is continuous without any finite-size effects detectable.