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

Abstract

The dynamic collective properties of the liquid Li 0.7 -Mg 0.3 alloy are studied by molecular-dynamics simulation. The dynamic structure factors and longitudinal current correlations at wave vectors between the hydrodynamic and kinetic regime (0.2ϽkϽ4 Å Ϫ1 ) are analyzed. In order to discuss the influence of the mass difference between particles on the longitudinal modes, the same alloy, except that the mass of the heavy atoms was increased by a factor of 10, was simulated. The resulting properties are compared with those of the ordinary Li 0.7 -Mg 0.3 alloy. It is shown that at wave vectors prior to the hydrodynamic region both fast and slow longitudinal modes of kinetic character propagate through light and heavy particles, respectively. In the hydrodynamic limit fast and slow modes merge into a single acoustic mode. It is corroborated that fast propagating modes in disparate mass liquid mixtures have a kinetic character. The number-number and concentration-concentration time correlation functions were also determined. The former shows a behavior analogous to that of the corresponding function in one-component liquids. The second reflects the existence of propagating concentration modes.

Viscoelastic model for the dynamic structure factors of binary systems

“…The interatomic pair potentials were obtained in the case of Li 0.7 Mg 0.3 within the neutral pseudoatom method [11] while in the case of Li 4 Pb were taken from Copestake et al [12]. The collective propeties of the Li 0.7 Mg 0.3 alloy have been studied by Anento et al [13,14], and those of Li 4 Pb by several authors [15,16], the latter being the first system where the fast sound phenomenon was observed, which consists in the appearance of a peak in the Li-Li dynamic structure factor, absent in the Pb-Pb one, with a very high frequency similar to that of pure Li. In this paper we include MD results obtained for both systems, in the case of Li 0.7 Mg 0.3 with 570 particles and in the case of Li 4 Pb with 600 particles for most k values and with 11000 particles for the smallest ones (k min = 0.10Å −1 ).…”

Dynamics of ternary liquid mixtures: Generalized collective modes analysis

“…This is connected with the fact observed previously for binary liquids in the GCM treatment [8] and the numerical analysis of MD data [12] that some branches of propagating collective excitations have a pronouncedly partial character in the short-wavelength region and can be simply obtained from the analysis of MD-derived partial current-current spectral functions. Hence, the simplest extension of the hydrodynamic sets (6) by taking into account just the first time derivatives of hydrodynamic variables would not permit to correctly describe collective excitations in short-wavelength region.…”

“…constructed in the same manner as in the case of transverse dynamics (see (12) and (13)). It is easy to check that both mass-concentration currents J x (k, t) are mutually orthogonal dynamic variables and both are orthogonal to the total mass-current density J t (k, t).…”

Collective dynamic properties in simple crystals: Influence of the structural disorder

“…Recent MD studies of liquid binary mixtures 18,20 have shown that, for a given k, the frequency associated with the light particles is in general higher than that for the heavy particles. However, for wavenumbers smaller than a given value (hydrodynamic regime) the frequencies for the light and heavy particles become equal so that L…”

“…21 However, a careful analysis of the origin of these peaks has shown that they should be attributed to the existence of modes of kinetic character analogous to the optic modes in ionic mixtures. 4,20,22,23 After the initial regime the qualitative features of the number density and partial relation dispersion are 1,2 The corresponding number density transverse current spectra C T NN ͑k , ͒ were then calculated according to Eq. (1).…”