Dynamic Structure of He-Ne Mixtures by Molecular Dynamics Simulation: From Hydrodynamic to Fast and Slow Sound Modes

Molecular dynamics (MD) results for the dynamic structure of a ${\mathrm{He}}_{0.77}{\mathrm{Ne}}_{0.23}$ gas mixture at two densities (15.8 and $36.1{\mathrm{nm}}^{-3\mathrm{}}$) show a clear crossover from hydrodynamic modes to distinct excitations for the two species. The higher density dispension curve neatly shows high- and low-frequency branches setting on with a rather localized transition. The lower density results agree very well with existing neutron scattering data and, in particular, display hydrodynamic behavior up to $k\approx 2{\mathrm{nm}}^{-1\mathrm{}}$, in contrast with the conclusions of previous simulation studies. A smooth transition to fast sound is shown to take place for $2<k/{\mathrm{nm}}^{-1\mathrm{}}<5\mathrm{}$, where the present MD data fill the existing gap in the experimental results.