Collective dynamics in molten potassium: an Inelastic X-ray Scattering study

TL;DR

This study uses inelastic X-ray scattering to explore high-frequency collective dynamics in molten potassium, revealing a two-step relaxation process and a maximum sound velocity exceeding hydrodynamic predictions, linked to microscopic disorder.

Contribution

It provides new insights into the high-frequency dynamics of molten potassium, extending previous neutron scattering results with inelastic X-ray scattering and identifying the role of microscopic disorder.

Findings

Discovered a two-step relaxation scenario in molten potassium.

Identified a maximum sound velocity exceeding hydrodynamic values.

Linked the phenomenology to microscopic disorder rather than phase crossover.

Abstract

The high frequency collective dynamics of molten potassium has been investigated by inelastic x-ray scattering, disclosing an energy/momentum transfer region unreachable by previous neutron scattering experiments (INS). We find that a two-step relaxation scenario, similar to that found in other liquid metals, applies to liquid potassium. In particular, we show how the sound velocity determined by INS experiments, exceeding the hydrodynamic value by $\approx 30 %$, is the higher limit of a speed up, located in the momentum region $1<Q<3$ nm$^{-1}$, which marks the departure from the isothermal value. We point out how this phenomenology is the consequence of a microscopic relaxation process that, in turn, can be traced back to the presence of ''instantaneous'' disorder, rather than to the crossover from a liquid to solid-like response.