Longitudinal collective modes in simple liquid binary alloys: A computer simulation study

The dynamic collective properties of the liquid Li-0.7-Mg-0.3 alloy are stu died by molecular-dynamics simulation. The dynamic structure factors and lo ngitudinal current correlations at wave vectors between the hydrodynamic an d kinetic regime (0.2<k<4 Angstrom (-1)) are analyzed. In order to discuss the influence of the mass difference between particles on the longitudinal modes, the same alloy, except that the mass of the heavy atoms was increase d by a factor of 10, was simulated. The resulting properties are compared w ith those of the ordinary Li-0.7-Mg-0.3 alloy. It is shown that at wave vec tors prior to the hydrodynamic region both fast and slow longitudinal modes of kinetic character propagate through light and heavy particles, respecti vely. In the hydrodynamic limit fast and slow modes merge into a single aco ustic mode. It is corroborated that fast propagating modes in disparate mas s liquid mixtures have a kinetic character. The number-number and concentra tion-concentration time correlation functions were also determined. The for mer shows a behavior analogous to that of the corresponding function in one -component liquids. The second reflects the existence of propagating concen tration modes.