On the observation of propagating sound modes at high momentum transfer in viscous liquids and glasses

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Abstract
The dynamic structure factors, S(Q,ω), at a range of scattering vectors (Q) in the range 0.2<Q <3.0 Å-1 are calculated in a molecular dynamics (MD) simulation of the viscous, network-forming liquid, ZnCl2. At intermediate Q (0.2<Q<0.5 Å-1), Brillouin features, with shifts and widths proportional to Q and Q2, respectively, are observed. These features persist above the frequency of the "boson peak," in apparent discord with some suggestions for its origin. At high values of Q, close to the first peak of the static structure factor, a Q -dependent feature is seen in the inelastic part of S(Q,ω). This is analogous to a recent experimental observation on the similar, network-forming system B2O3 which was assigned to a "sound mode" contribution. An instantaneous normal mode (INM) analysis was applied to understand the relationship between these observations and the nature of the underlying modes of the liquid. The INM analysis accurately reproduces the observed (in MD) behavior of S(Q,ω) but does not support the existence of well-defined, plane-wave-like sound modes in the fluid in the relevant régimes of Q and ω. Alternative explanations of the origin of the Brillouin and S(Q)-dependent features are proposed on the basis of the INM results, and results related to the origin of the boson peak in the density of states are presented. © 1998 American Institute of Physics