Effect of information and communications technology on development

Thesis(Master) --KDI School:Master of Public Policy,2012masterpublishedHyangsuk Seong

Melting of a repulsive screened Coulomb system in two dimensions: Effect of corrugation

By use of constant energy molecular dynamics simulations, we have investigated the melting and freezing transitions in a two-dimensional system consisting of a constant density of classical particles interacting with a repulsive screened Coulomb (Yukawa) potential. In particular, we have investigated the role of an incommensurate substrate corrugation potential of sixfold symmetry on these transitions by probing the temperature dependence of the bond orientational order parameter (ψ6) and the corresponding susceptibility (χ6). Other physical quantities such as energy, diffusion constant, and the density of local topological defects have been monitored through the transition region. In the absence of a corrugation potential the system shows a sharp melting transition. In the presence of a corrugation potential the transition temperature increases and the transition becomes smoother. In contrast to the corrugation-free case we find a broad peak in χ6 above the transition temperature. We interpret this behavior in terms of the melting of a domain-wall solid. The detailed nature of this smooth transition depends on the degree of incommensurability. Relevance of these results to stage-2 graphite intercalation compounds is discussed

Domains and domain walls in graphite intercalation compounds

The structure of domains and domain walls in the low-temperature solid phase of stage-2 alkali-metal graphite intercalation compounds is studied using molecular-dynamics (MD) simulation and a model potential obtained from fitting to the liquid structure data. For RbC24, we find small domains of (√7 × √7 ) structure containing mostly seven Rb atoms and with domain walls consisting of (2×2) and (2×3) elemental plaquettes. These nanostructures provide a clear picture of the discommensurations and a consistent understanding of the x-ray-diffraction measurements. Based on the simulation results, we propose a periodic domain-wall model that explains very well the dominant features of the x-ray structure factor for both RbC24 and CsC24 when we allow for small changes in the planar density