Coexistence of hexatic and isotropic phases in two-dimensional Yukawa systems

Abstract

We performed Brownian-dynamics simulations on the melting of two-dimensional colloidalcrystals in which particles interact via a Yukawa potential. A stable hexatic phase was found inthe Yukawa systems, but we also found that the melting of Yukawa systems is a two-stagemelting, which is inconsistent with the Kosterlitz–Thouless–Halperin–Nelson–Young(KTHNY) theory. A two-phase coexistence region between the stable hexatic phase and theisotropic liquid phase was found. The behavior of point defects in the coexistence region isvery complicated. The emergence of some unstable free disclinations and grainboundaries was a characteristic representative of the isotropic liquid phase, and alarge number of free dislocations indicated the existence of a hexatic phase. Thisindicates the existence of a phase of hexatic–isotropic liquid phase coexistence.The big picture in the melting of a two-dimensional Yukawa system is that firstthe system undergoes a transition induced by the formation of free dislocations,then it goes through a phase coexistence, and finally, it comes into an isotropicfluid phase. This melting process is consistent with experiments and simulations.