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 nm^-3 shows clear side peaks or shoulders at frequencies {ω}=ωₛ⁽¹⁾(k) for wave numbers k up to {}15 nm^-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.