The temperature dependent collective dynamics of liquid Rubidium

Liquids alkali metals show near the melting point at small momentum transfer values an upward bending of the dispersion relation. This so-called positive dispersion can be described within generalised hydrodynamics as a visco-elastic reaction of the liquid. There is a speculation that long-living clusters could be the physical reason behind this phenomenon. To shed light on this question a coherent inelastic neutron scattering experiment on liquid Rubidium was performed at four temperatures starting at the melting point. Distinct deviations from molecular dynamics simulation results occur at the lowest accessible momentum transfer values. With rising temperature the excitation frequencies soften at a momentum transfer value which is correlated with the dimensions of the supposed clusters.