The classification of order-disorder transitions on surfaces

Order-disorder transitions occur in adsorbed systems of great diversity.However, the transitions themselves can be shown to fall into just a few classes which are characterized by differing critical properties. The particular class to which a given transition belongs is determined directly from the nature of the ordered state and substrate. The critical behavior of some of these classes is of great interest currently, and the observations of such transitions provide a test of the modern theory of critical phenomena. The simple but powerful ideas of Landau and Lifshitz which underlie the above classification are elucidated in the context of order-disorder transitions on surfaces. The “Landau rules” are derived, applied, and yield the several universality classes which can be observed in surface transitions. Specific applications are made to recent experiments, and some areas which call for experimental investigations are noted.