Efficient analytical expressions for dynamic structure of liquid binary alloys: K–Cs as a case study

Abstract

A fitting scheme for analysis of collective dynamics in liquid binary alloys is proposed. It is based on explicit treatment of contributions from three relaxing modes and two types of propagating modes to the partial density-density time correlation functions and corresponding partial dynamic structure factors. Exact sum rules for each partial dynamic structure factor were taken into account. The proposed fitting scheme was applied to the liquid equimolar K-Cs alloy. Analysis of simulation-derived partial time correlation functions as well as of their corresponding Bhatia-Thornton 'number-concentration' combinations allowed dispersion and damping of the two branches of collective excitations and the behaviour of relaxing modes in a wide range of wave numbers to be obtained. A comparison with the inelastic neutron-scattering intensities for the liquid K-Cs alloy was performed.

Analyzing the dynamic structure of liquid metals and alloys

“…Looking at the parameters of these modes (not shown here, please refer to [3]), considering first their frequencies, the low frequency is very similar to the corresponding pure Cs branch. On the other hand, the high frequency branch has only remote resemblance with the pure potassium branch and, notably, it tends to a non-zero limit as q tends to zero.…”

“…Propagating modes usually contribute to side peaks in S (q, ω) while relaxing modes contribute to the central peak. Looking at F(q, t), the propagating ones appear as oscillations while the relaxing ones contribute as exponential decays [3].…”

“…The positive dispersion is a largely observed phenomenon [5] that can be rather high and reach 15% in the case of silicon. In Figure 1, we have plotted the dispersion curves corresponding to our studies of K [2] and Cs [3]. These results were obtained from simulations with 2048 particles in a thermodynamic state close to the melting point.…”

“…Results for liquid K-Cs alloy at equiatomic composition [3] are presented in Figure 2. The partial dynamic structure functions were fitted considering two relaxation and two propagation modes.…”

“…In this work, a recently developed analysis scheme [1][2][3] inspired from the Generalized Collective Modes (GCM) approach [4] is presented and applied to simulation results of liquid metals and alloys. We focus on three features revealed by increasing accuracy of available data, namely the positive dispersion of acoustic waves in pure liquids, the so-called "fast sound" effect in mixtures and finally the possible coupling between transverse and longitudinal excitations.…”