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

Abstract

The total dynamics structure factor S(k,\ensuremath{\omega}) measured by neutron scattering on a mixture of 80% He and 20% Ne at 39.3 K and total number density 15.1 ${\mathrm{nm}}^{\mathrm{\ensuremath{-}}3}$ shows clear side peaks or shoulders at frequencies \ensuremath{\omega}=${\mathrm{\ensuremath{\omega}}}_{\mathrm{s}}^{(1)}$(k) for wave numbers k up to \ensuremath{\approxeq}15 ${\mathrm{nm}}^{\mathrm{\ensuremath{-}}1}$. $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.