Neutron Brillouin Scattering Study of Collective Dynamics in a Dense He-Ne Gaseous Mixture

The dynamic structure factor S(Q,w) of a 77% He and 23% Ne gaseous mixture at T=39.3 K and total number density n = 15.8 nm^-3 has been measured by inelastic neutron scattering at small angles. In the range of wave vectors studied, we find clear side peaks at frequencies following the hydrodynamic sound dispersion c k of the binary mixture (with c the adiabatic sound speed in the mixture). Surprisingly, neither fast modes nor a transition from normal to fast sound are observed in the above low-k region, indicating that hydrodynamic behavior persists up to k values which are larger than those predicted by theories and computer simulations.