SINGLE-PARTICLE MOTION IN A DENSE LIQUID - A COMPETITION OF MODE-COUPLING EFFECTS

In two quasi-elastic neutron scattering experiments on liquid sodium a t 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 ligh t of a new theory, the mode coupling theory, and reveal two clearly di fferent mode coupling effects in the liquid: Coupling to collective de nsity fluctuations-hindering a particle's diffusion, and coupling to t ransverse shear modes, which promote single particle dynamics, The int erplay of these two basic mechanisms leads to a new understanding of s ingle particle motion in simple liquids, being governed by the tempera ture dependent competition of two mode coupling effects. In conclusion the non-trivial temperature dependence of the diffusion coefficient i n simple liquids can be understood on a purely microscopic basis witho ut phenomenological ingredients.