Dependence of the Atomic Level Green-Kubo Stress Correlation Function on Wavevector and Frequency. Molecular Dynamics Results from a Model Liquid

TL;DR

This study investigates how atomic-level stress wave propagation in liquids depends on wavevector and frequency, revealing insights into viscosity, stress wave ranges, and their relation to temperature and vibrational dynamics.

Contribution

It introduces a Fourier transform-based analysis of atomic stress correlations to connect stress wave propagation characteristics with frequency, wavevector, and temperature in liquids.

Findings

Stress wave propagation ranges extend beyond nearest neighbors at low frequencies.

Increased low-frequency viscosity correlates with longer stress wave propagation.

Crossover from localized to propagating stress behavior occurs near the Boson peak frequency.

Abstract

We report on a further investigation of a new method that can be used to address vibrational dynamics and propagation of stress waves in liquids. The method is based on the decomposition of the macroscopic Green-Kubo stress correlation function into the atomic level stress correlation functions. This decomposition, as was demonstrated previously for a model liquid studied in molecular dynamics simulations, reveals the presence of stress waves propagating over large distances and a structure that resembles the pair density function. In this paper, by performing the Fourier transforms of the atomic level stress correlation functions, we elucidate how the lifetimes of the stress waves and the ranges of their propagation depend on their frequency, wavevector, and temperature. These results relate frequency and wavevector dependence of the generalized viscosity to the character of propagation of the shear stress waves. In particular, the results suggest that an increase in the value of the frequency dependent viscosity at low frequencies with decrease of temperature is related to the increase in the ranges of propagation of the stress waves of the corresponding low frequencies. We found that the ranges of propagation of the shear stress waves of frequencies less than half of the Einstein frequency, extend well beyond the nearest neighbor shell even above the melting temperature. The results also show that the crossover from quasilocalized to propagating behavior occurs at frequencies usually associated with the Boson peak.