Atomic Dynamics in Simple Liquid: de Gennes Narrowing Revisited

Abstract

The de Gennes narrowing phenomenon is frequently observed by neutron or x-ray scattering measurements of the dynamics of complex systems, such as liquids, proteins, colloids, and polymers. The characteristic slowing down of dynamics in the vicinity of the maximum of the total scattering intensity is commonly attributed to enhanced cooperativity. In this Letter, we present an alternative view on its origin through the examination of the time-dependent pair correlation function, the van Hove correlation function, for a model liquid in two, three, and four dimensions. We find that the relaxation time increases monotonically with distance and the dependence on distance varies with dimension. We propose a heuristic explanation of this dependence based on a simple geometrical model. This finding sheds new light on the interpretation of the de Gennes narrowing phenomenon and the α-relaxation time.

Atomic Dynamics in Real Space and Time

“…Since then it became customary to equate the observation of the de Gennes narrowing to the confirmation of collective excitations. A recent study [18], however, showed otherwise. It was found that even in high-temperature liquid, in which atomic motions are uncorrelated, the decay time of G(r, t) depends linearly on distance.…”

“…The data points are shown in the form of log{[τ(r)-τ 0 ]/τ r } versus log(r/r 1 ) to highlight χ from the expected power law dependence. The short dashed lines serve as guides to the eye[18].…”

Local Density Correlations in Liquids

“…This effect is generally considered to be the evidence of collective excitations, but our recent results have shown otherwise. As shown in Figure 2 the decay time of the VHF increases linearly with distance, with the rate related to the Maxwell relaxation time [4]. In general the r-dependent decay time depends on r as,…”

“…The VHF has been known for a long time, but only recently it became possible to determine it by inelastic scattering experiments as shown in Figure 1 [3]. At the same time the behavior of the VHF was examined by molecular dynamics (MD) simulation, which gave a new insight on the interpretation of the VHF [4]. In particular, it became clear that the dynamics of the medium-range correlations beyond the first nearest neighbors is quite distinct from that of the short-range structure represented by the first neighbor shell.…”

“…Linear dependence of the decay time of the Van Hove function on distance, simulated for liquid Fe[4].…”