CRITICAL PHENOMENA AND DEFECTS IN PHYSISORBED LAYERS

Structural phase transitions in two-dimensional systems are discussed with emphasis on transitions in which topological defects like domain walls, dislocations or steps have a key position. Experimentally, adsorbed monolayers on single crystal surfaces provide model systems for the realization of these phase transitions. We will discuss here some aspects of the structural phase transitions of Xe monolayers on Pt(111), Gr(0001) and Ag(111). The Xe{single bond}Pt(111) system exhibits a rich variety of 2D solid phases and corresponding mutual transitions, driven by the energetics and dynamics of domain walls in the Xe lattice. These domain wall defects are caused by the delicate interplay of the Xe{single bond}Xe and Xe{single bond}Pt interactions. Xe monolayers on Gr(0001) and Ag(111) have been discussed as model systems of dislocation induced melting, according to the Kosterlitz-Thouless-Halperin-Nelson-Young theory. We will show here that the corrugation of the substrate may indeed be responsible for the observation of a hexatic structure factor in the molten Xe monolayers, shedding some doubt on the intrinsic hexatic character of these liquid films. In addition to defects intrinsic to the adlayer, extrinsic defects due to the limited coherence of the substrate surface (finite step density) or due to small amounts of adsorbing impurities can substantially influence two-dimensional phases and their transitions. As an example we will discuss the impurity-quenched orientational epitaxy of Kr and Ar monolayers on Pt(111). © 1990 Pergamon Press plc.