Melting In Two-dimensional Yukawa Systems

In this thesis, we present a Brownian-dynamics simulation on a 2D Yukawa system and study the melting of 2D charged colloidal crystals.Firstly, we give the basic knowledge and the existing results about 2D colloidal crystals melting. A well-known theory about 2D melting is Kosterlitz-Thouless-Nelson-Halperin-Young(KTHNY) theory. According to KTHNY theory, the melting of a two-dimensional system is a two-stage continuous transition through the so-called Hexatic phase. In recent years, a large number of experiments and computer simulations have indicated that there is, indeed, a two-stage melting scenario in two-dimensional sys-tems as prescribed by the KTHNY theory. However a different theoretical approach, evoking grain-boundary-induced melting, is a first-order transi-tion suggested by Chui.Secondly, We introduce our model and present the results. We per-form Brownian dynamics simulations on the melting of two-dimensional colloidal crystals in which particles interact via a Yukawa potential. A stable hexatic phase is found in the Yukawa systems, but we also find that the melting of Yukawa systems is a two-stage melting, which is inconsistent with the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. A two-phase coexistence region between the stable hexatic phase and the isotropic liquid phase is found. The behavior of point defects in the coex-istence region is very complicated. The emergence of some unstable free disclinations and grain boundaries is a characteristic representative of the isotropic liquid phase, and a large number of free dislocations indicate the existence of a hexatic phase, which illustrates 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 firstly the system undergoes a transition induced by the formation of free dislocations, then it goes through a phase coexistence. and finally, it comes into an isotropic fluid phase.At last, we make a conclusion and an outlook of this thesis.