Crossover between hydrodynamic and kinetic modes in binary liquid alloys

TL;DR

This study uses inelastic x-ray scattering to explore the transition from hydrodynamic to kinetic modes in liquid Na-K alloys, revealing positive dispersion caused by relaxation processes, not fast sound phenomena.

Contribution

It provides experimental evidence of positive dispersion in binary liquids due to relaxation, clarifying the crossover between hydrodynamic and kinetic modes.

Findings

Positive dispersion occurs below the crossover wavelength.

The dispersion is due to relaxation processes, not fast sound.

Two phonon-like modes coexist in the liquid alloy.

Abstract

Inelastic x-ray scattering (IXS) measurements of the dynamic structure factor in liquid Na57K43, sensitive to the atomic-scale coarse graining, reveal a sound velocity value exceeding the long wavelength, continuum value and indicate the coexistence of two phonon-like modes. Applying Generalized Collective Mode (GCM) analysis scheme, we show that the positive dispersion of the sound velocity occurs in a wavelength region below the crossover from hydrodynamic to atom-type excitations and, therefore, it can not be explained as sound propagation over the light specie (Na) network. The present result experimentally proves the existence of positive dispersion in a binary mixture due to a relaxation process, as opposed to fast sound phenomena.