Longitudinal, transverse, and single-particle dynamics in liquid Zn: Ab initio study and theoretical analysis

We perform ab initio molecular dynamics simulations of liquid Zn near the melting point in order to study the longitudinal and transverse dynamic properties of the system. We find two propagating excitations in both of them in a wide range of wave vectors. This is in agreement with some experimental observations of the dynamic structure factor in the region around half the position of the main peak. Moreover, the two-mode structure in the transverse and longitudinal current correlation functions had also been previously observed in high pressure liquid metallic systems. We perform a theoretical analysis in order to investigate the possible origin of such two components by resorting to mode-coupling theories. They are found to describe qualitatively the appearance of two modes in the dynamics, but their relative intensities are not quantitatively reproduced. We suggest some possible improvements of the theory through the analysis of the structure of the memory functions. We also analyze the single-particle dynamics embedded in the velocity autocorrelation function, and explain its characteristics through mode-coupling concepts.