Fast sound in two-component liquids

Abstract

A new high-frequency and short-wavelength collective mode specific to binary liquid mixtures with large mass difference is observed in a computer simulation of Lio.8Pbo.2 and discussed within the framework of the Mori-Zwanzig formalism. The mode shows linear dispersion in a wave-number regime 0.1 A "* < ^ < 0.6 A~^ but its propagation velocity is higher than the ordinary sound velocity by more than a factor of 3. Its attenuation is only weakly q dependent in contrast to the damping of ordinary sound. In Lio.8Pbo.2 "fast sound" entails motion of the lighter atoms only.PACS numbers: 61.25.MvWe report the existence of an additional propagating collective mode in binary liquid mixtures, confined to high frequencies and large wave numbers well beyond the hydrodynamic regime. It can be observed in inelastic neutron-scattering experiments or in computer simulation studies of two-component systems with large atomic-mass difference. Some of these systems may respond to a high-frequency short-wavelength perturbation with a density wave, which is supported by the light particles alone, essentially, without the heavy particles participating in the collective motion. Since the dispersion law of this excitation mode is much steeper in the linear region than that of the Brillouin peak of ordinary sound, we call the new mode "fast sound." We have observed fast sound in a computer simulation study ^ of a liquid alloy system of 250 particles in a periodic cell modeling Lio.gPbo.i^ at temperature r=1085 K and total number density w =0.045 58 A ~l Results for the partial dynamic structure factorsSss '(q\(o)

Longitudinal collective modes in simple liquid binary alloys: A computer simulation study

“…Since the pioneer MD simulation of Na-K liquid alloy 1 the majority of studies have been centered on the alloy Li 4 Pb. In a MD simulation of this system Bosse et al 2 observed the existence of high-frequency peaks in the partial Li-Li dynamic structure factors at rather large wave vectors. These peaks were associated with longitudinal modes that propagate through light particles with a velocity much higher than that of the ordinary sound ͑the so-called ''fast sound''͒.…”

Longitudinal Dynamics in a Lennard-Jones Binary Liquid: Crossover From Hydrodynamics to the Molecular Regime

“…So far it has not been finally established what is going on with the high-frequency propagating branch in the long-wavelength limit. Hence, the problem of "fast sound" [7] reported in the literature over almost 20 years is still far from being finally solved. It is not also known what is the role of concentration in the damping of high-frequency branch, how the mass ratio affects the width of hydrodynamic region, how will the dispersion law change by decreasing the concentration of relevant species up to the impurity limit.…”

Microscopic dynamics in liquid binary alloys: orbital-free ab-initio molecular dynamics studies

“…On the theoretical side, kinetic theory calculations [18] for two-component fluids with large atomic mass difference, have confirmed the appearance of a collective mode at high frequencies and wave numbers q q h . Moreover, this mode propagated with a phase velocity close to that of the pure light component, which was clearly greater than the hydrodynamic sound velocity, c h , of the two-component system.…”

“…It was followed by the CMD results for liquid Li 4 Pb [7], where a new, high-frequency mode, allegedly supported by the light Li atoms only, (the so-called "fast sound") was found. This finding spurred a wealth of experimental [8][9][10][11][12][13], computer simulation [8,[14][15][16][17] and theoretical [18][19][20][21][22] works focused on the investigation of the nature and properties of the collective excitations in liquid binary systems.…”