Thermal Fluctuations in Equilibrium and Their Modification by Surface Defects

Abstract

Thermally activated processes like temperature-dependent changes of sites and the mapping of phase diagrams give essential information about the energy differences between sites and about lateral interactions. We show examples of atomic adsorbates (H, O, S) on close packed Ni(111) and Ru(0001) surfaces investigated quantitatively with LEED. The information about lateral correlations of critical fluctuations contained in spot profiles close to a continuous phase transition is illustrated in the example of the order-disorder transition of (√3×√3)R 30° -Au/Si(111). Here the critical exponents β, γ and ν of the order parameter, the susceptibility and the correlation length were determined for the unique situation of thermal equilibrium between 3-dimensional gold islands and the 2-dimensional (√3×√3)R 30° phase, and shown to belong to the 3-state Potts universality class. Impurity induced changes of phase diagrams and, more severely, of critical properties of continuous phase transitions in two-dimensional systems are exemplified by adsorbed atomic hydrogen on Ni(111). Although strongly chemisorbed, the phase diagram of this system is sensitive to isotopic mixtures. Finite energies of mixing and changes of the order-disorder phase transition of the (2 × 2) phase from continuous to first order were found. The critical properties of this order-disorder transition are changed by the presence of preadsorbed oxygen concentrations of 0.3% to 3% of a monolayer. The result is a crossover from 4-state Potts exponents far away from T c to Ising-like exponents closer to T c .