Abstract
We present evidence for changes in the dynamics of liquid rubidium with rising temperature. The thermal expansion of this liquid alkali metal shows a changing derivative with temperature in a temperature range of about 400-500 K. With neutron scattering the amplitude at the structure factor maximum demonstrates a changing slope with increasing temperature. A derived averaged structural relaxation time can be understood that an additional relaxation process sets in upon cooling. The deduced generalized viscosity and high frequency shear modulus indicate a change in dynamics in the same temperature range. All these findings point to a change in dynamics of the equilibrium liquid metal state with a dynamical crossover from a viscous to a fluid-like liquid metal well above the melting point.
Highlights
During the past decades the collective dynamics of liquid metals has been intensively investigated near the melting point, e.g. for an earlier and a more recent review on the dynamics of liquid metals see [1, 2]
It appears that the correlations between volume and entropy fluctuations are changing much more in a temperature range up to about 400-500 K compared to the higher temperatures
The average relaxation time of density fluctuations at this momentum transfer follows the same trend. An explanation for this change in dynamics might be the set-in of an additional slow relaxation process upon cooling, which has been related to structural freezing
Summary
During the past decades the collective dynamics of liquid metals has been intensively investigated near the melting point, e.g. for an earlier and a more recent review on the dynamics of liquid metals see [1, 2]. Further investigations on changes in the dynamics with rising temperature up to the boiling point and beyond focused on the self-diffusion of liquid alkali metals and the underlying changes in the dynamics with decreasing density [8]. The alkali metal rubidium has been studied for several decades and a lot of data, macroscopic and microscopic, are available over a wide temperature range from experiment and simulation (e.g.[13,14,15,16,17]). Rubidium represents an ideal test case to investigate changes in liquid metal dynamics with rising temperature in detail.
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