Experimental evidence for the long-time decay of the velocity-autocorrelation in liquid sodium

Incoherent inelastic neutron-scattering experiments on liquid sodium at high temperature revealed that atomic motions in simple liquids are governed by hydrodynamic shear modes leading to measurable deviations from Fick's law of diffusion. The experiments for the first time verify earlier predictions of a ``long-time tail'' behavior of the velocity-autocorrelation function of liquid particles as derived from computer simulation data and theory. A proper analysis of the experimental data demonstrates the existence of a corresponding low-frequency cusp in the velocity-autocorrelation spectrum.