Dynamic structure of He-Ne mixtures by MD simulation: from hydrodynamic to fast and slow sound modes

Molecular dynamics results for the dynamic structure of a He0.77Ne0.23 gas mixture at two densities (15.8 nm-3 and 36.1 nm-3) are presented. A clear description of the crossover from hydrodynamic modes to distinct excitations for the two species is obtained. The higher density data neatly show high- and low-frequency branches setting on in the dispersion curve, with a rather localized transition. The lower density results remarkably agree with existing neutron scattering data and, differently from previous simulation studies, display hydrodynamic behavior up to k = 2 nm-1. A smooth transition to fast sound is shown to take place for 2 < k / nm-1 < 5, where the present MD data fill the existing gap in the experimental results.