Molecular dynamics study of two-dimensional melting transition in copper via the embedded atom method

Abstract

We report detailed molecular dynamics results for the two-dimensional (2D) melting transition using embed atomic method. The simulations are performed in the $NPT$ ensemble. Under different temperatures, the orientational correlation functions ${g}_{6}(r)$ and the pair distribution functions are measured and compared with the prediction of KTHNY theory. The size effect of distribution of the second moment of the local bond-orientational order parameter ${\ensuremath{\mid}{\ensuremath{\psi}}_{6,i}\ensuremath{\mid}}^{2}$ is studied. All results prove the existence of an intermediate hexatic phase during 2D melting. The dependence of average potential energy per atom on temperature and evidences from observations on the global bond-orientational order parameter and global translational order parameter show that the melting contains two stages and is a first-order transition as a whole. By exploring the evolution of topological defects, along with observations on the distributions of defects in inherent structures of typical crystalline phase, hexatic liquid, and isotropic liquid, we find that, instead of disclinations, the formation of defect chains and the correlation between them play an important role in both steps, which is rather different from the prediction of KTHNY theory.