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Cover Picture: Melting of Crystals in Two Dimensions (ChemPhysChem 5/2010)
Artist's view of a two-dimensional (2D) hexagonal crystal containing dislocations, which are given by pairs of five- (red) and seven-fold (green) coordinated particles, as described by U. Gasser et al. on p. 963. As highlighted by the red lines, two lattice lines end at each dislocation. The appearance of such topological defects in the crystal causes the melting of 2D crystals, as was predicted in the 1970s by Kosterlitz, Thouless, Halperin, Nelson, and Young. Close to the melting transition, dislocations appear in pairs and their unbinding is the cause for the transition from the crystal to the hexatic phase with quasi-long-range orientational but short-range translational order. At a higher temperature, dislocations unbind into disclinations, isolated five- or seven-fold coordinated particles. In this second transition, the orientational order is lost and the system reaches the isotropic liquid state. The first conclusive experimental evidence for these two transitions from the 2D crystal to the liquid was obtained from imaging experiments on colloidal model systems.