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 He_0.77Ne_0.23 gas mixture at two densities (15.8 and 36.1 nm^-3) 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~2 nm^-1, in contrast with the conclusions of previous simulation studies. 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.