Dynamics in He at 4 and 8 K from inelastic neutron scattering

From inelastic neutron scattering we determine the dynamic structure factor S(q, omega ) of fluid He at 4 K (1 bar; 19.51 nm super(-3)) and 8 K (18.7 bar; 19.46 nm super(-3)) as a function of frequency omega for wavenumbers 1 < q < 30 nm super(-1). For q less than or approximate to 4 nm super(-1), S(q, omega ) is given by the Rayleigh-Brillouin triplet of hydrodynamics. For q > 4 nm super(-1) the central Rayleigh line vanishes and S(q, omega ) is given by the phonon Damped Harmonic Oscillator model similar as in superfluid helium. The phonons are overdamped for a small region near q identical with 20 nm super(-1). At 8 K, helium starts to behave in a visco-elastic manner, similar as seen in classical fluids. We discuss a unifying model that connects the properties of superfluid helium to those of classical fluids.