Phase transitions and critical phenomena in strongly chemisorbed adlayers: Influence of defects

Phase diagrams and, more severely, critical properties of continuous phase transitions in two-dimensional systems can strongly be changed by the presence of defects on the surface. We focus on the example of adsorbed atomic hydrogen on Ni(lll). 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 x 2) phase from continuous to first order were found. The critical properities 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. These effects can well be separated from finite size effects, which are present in addition. The influence of steps on the critical properties is particularly interesting, ii the order of the adsorbate is not interupted by steps. In this case, a similar change of critical exponents is seen, as for O/Ru(0001)-p(2 x 2). These results are discussed in the framework of random disorder.