High-Frequency Dynamics in a Molten Binary Alloy

Abstract

Heavily damped excitations are found in molten Li 4 Pb by inelastic neutron scattering. The experiment covered a kinematic range which enabled an unambiguous characterization of such excitations by means of the study of the wave vector dependence of their frequencies, lifetimes, and signal amplitudes. It is shown that the excitations being sampled exhibit features which substantially deviate from those expected for the propagation of an acoustic mode (which should involve in-phase atomic displacements). [S0031-9007(98) [7,8], and even some semimetallic liquids [9]. The most salient feature of such dynamic phenomena concerns their frequencies, which are well above those expected for a continuation to large wave vectors of hydrodynamic sound. In fact, kinetic-theory predictions [6] portray such excitations as being supported by the light component only so that they apparently travel with phase velocities close to those characteristic of the pure component, which are well above those given by the elastic constants of the mixture.Results from experiment and computer simulation on a variety of systems [1][2][3][4][5][6][7][8] can show well defined, heavily damped or overdamped features in S͑Q, v͒. If the motions being sampled are heavily damped or overdamped, only broad shoulders are visible, so that characteristic frequencies are usually obtained from peaks in the generalized susceptibility x͑Q, v͒~vS͑Q, v͒, or in the longitudinal current correlation function J 1 ͑Q, v͒ v͞Q 2 x͑Q, v͒ (for a comment on the meaning of such frequencies, see Ref.[10]). Such frequencies v Q are oftentimes converted to phase velocitieshv Q ͞Q. From the exploration of the phase velocity trend within the low-Q region, the presence of a mode of acoustic nature (where atoms involved execute in-phase displacements) propagating with a velocity well above that corresponding to hydrodynamic sound, has been inferred.The points which still are subject to controversy regard (a) the adequacy of discussing the nature on excitations appearing at relatively large-Q in terms of constructs which only retain full sense within the realm of hydrodynamics (i.e., a sound mode), and (b) the assignment of the observed frequencies, to a definite underlying microscopic mechanism, in the absence of further information such as the Q dependence of the excitation amplitude (which provides a direct insight into the phase relationships of the motions being sampled at a given frequency). Observation of clear nonacoustic modes at high frequencies was reported years ago for molten salts [4]. It was rationalized in terms of opticlike vibrations, but it has hardly ever been discussed within the present context. Also, excitations with frequencies well above hydrodynamic sound not involving propagation of longitudinal sound have been found in experiments in some molecular [7] or even metallic [9] liquids.The main thrust behind the substantial experimental effort witnessed in recent times for the scrutiny of this class of systems [1,2,5] was motivated by predictions made from ...

Microscopic collective dynamics in liquidpara−H2

“…The departure from full classical behavior is also attested by the large value found for the mean-square displacement, since a large portion of it is attributable to zero-point motions. The finding of strong anomalous dispersion contrasts with phenomena observed in more complex fluids such as molten alloys or water, 26 where the dispersion is known to arise from an admixture of modes of different character. The presence of such relatively long-lived excitations thus provides a microscopic explanation for the anomalous behavior observed in its heat and momentum-transport properties.…”

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

“…Recently, MD simulation has been extensively applied to the study of both the structure and the dynamic properties of liquid metals whereas liquid alloys have received less attention, especially in the case of dynamic collective properties. 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.…”

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

“…Several theoretical and CMD results for He-Ar and He-Ne mixtures have suggested that either the fast and slow sound merge into the hydrodynamic sound [8,14], or that the fast sound disappears at q h and the slow sound merges into the hydrodynamic one with a predicted value [19,20] of q h ≈ 0.07Å −1 . A similar value for q h has recently been obtained for liquid Li 4 Pb by both INS and by CMD calculations [11,15]. Recent CMD calculations [16], which followed the INS measurements of Bafile et al [12], for the He 0.77 Ne 0.23 gas mixture at two densities, showed a clear crossover from hydrodynamics to fast and slow modes at 0.2 q/Å −1 0.5; moreover, it was obtained that q h ≈ 0.2, which is substantially greater than previous estimates.…”

“…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.…”