Onset of depature from linearized hydrodynamic behavior in argon gas studied with neutron Brillouin scattering

We have investigated the dynamic structure factor of room-temperature ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ at two densities (0.25 and 0.63 times the critical) and at very low momentum transfer (0.3<κ<1.3 ${\mathrm{nm}}^{\mathrm{-}1}$). Linearized hydrodynamics agrees with the experimental data only at the higher density and κ<1 ${\mathrm{nm}}^{\mathrm{-}1}$. However, the description in terms of three extended hydrodynamic modes is fully satisfactory at both densities over the whole κ range. We provide the first experimental evidence that the concept of extended hydrodynamic modes is valid for the whole range from hydrodynamics up into the kinetic regime.