Fast sound in a helium-neon mixture determined by neutron scattering

The total dynamics structure factor S(k,ω) measured by neutron scattering on a mixture of 80% He and 20% Ne at 39.3 K and total number density 15.1 ${\mathrm{nm}}^{\mathrm{-}3}$ shows clear side peaks or shoulders at frequencies ω=${\mathrm{\omega }}_{\mathrm{s}}^{\mathrm{}\left(1\right)\mathrm{}}$(k) for wave numbers k up to ≊15 ${\mathrm{nm}}^{\mathrm{-}1}$. $omega sub s sup (1) (k)>ck— (with c the velocity of sound) in agreement with the predicted existence of a fast sound mode in mixtures, due to oscillations in the light component (1) alone.