Single particle motion in a dense liquid: A competition of mode coupling effects

Abstract

In two quasi-elastic neutron scattering experiments on liquid sodium at 380 K and 900 K at saturated vapour pressure, single particle motion has been studied with high precision. Special emphasis was placed on the different single particle dynamics near the melting point and the boiling point of the liquid. The experiments are discussed in the light of a new theory, the mode coupling theory, and reveal two clearly different mode coupling effects in the liquid: Coupling to collective density fluctuations-hindering a particle's diffusion, and coupling to transverse shear modes, which promote single particle dynamics. The interplay of these two basic mechanisms leads to a new understanding of single particle motion in simple liquids, being governed by the temperature dependent competition of two mode coupling effects. In conclusion the non-trivial temperature dependence of the diffusion coefficient in simple liquids can be understood on a purely microscopic basis without phenomenological ingredients.