Universal acoustic dispersion in liquid alkali metals

The high-frequency acoustic dynamics of liquid cesium has been investigated by inelastic x-ray scattering as a function of pressure up to 5 GPa, corresponding to a volume compression of 50%. The obtained acoustic dispersion curves are found to scale on a single master curve, which turns out to be followed also by all the data available in the literature for liquid alkali metals, spanning a very large density range. This shows that the high-frequency acoustic modes of these systems are largely unaffected by the details of the relaxation dynamics characteristic of the macroscopic limit. Moreover, we find that a common model of interatomic potential describes them with good approximation for very different packing fractions and electronic screening contributions.