Density dependent single particle motion in liquid sodium

Abstract In a series of quasielastic neutron scattering experiments on liquid sodium the density dependence of the single particle motion was investigated for a variety of states along the liquid vapor co-existence curve. The results are in excellent agreement with computer simulation studies on hard sphere systems and can – on a quantitative basis – be discussed in the light of mode-coupling theory. A distinct universal deviation for the density dependence of the diffusion constant is found, which can be understood on a purely microscopic basis: at high density, the particle's motion is hindered by a coupling to longitudinal density fluctuations, while at elevated temperatures and intermediate densities a coupling to transverse shear modes sets in, promoting single particle dynamics relative to Brownian Diffusion. At even higher temperatures and low liquid densities mode-coupling effects die out and the system gradually returns to simple binary collision dynamics.