Direct evidence for strong crossover of collective excitations and positive sound dispersion in the supercritical state

TL;DR

This study provides direct evidence of the crossover in collective excitations and positive sound dispersion in supercritical fluids, showing a transition at the Frenkel line that separates regimes with and without transverse modes.

Contribution

It demonstrates the existence and disappearance of transverse modes and positive sound dispersion in supercritical fluids across the Frenkel line using molecular dynamics simulations.

Findings

Transverse modes exist below the Frenkel line but vanish above it.

Positive sound dispersion is present below the Frenkel line and disappears above it.

The Frenkel line marks a clear boundary in the supercritical phase diagram.

Abstract

Supercritical state has been viewed as an intermediate state between gases and liquids with largely unknown physical properties. Here, we address the important ability of supercritical fluids to sustain collective excitations. We directly study propagating modes on the basis of correlation functions calculated from extensive molecular dynamics simulations, and find that the supercritical system sustains propagating solid-like transverse modes below the Frenkel line but becomes devoid of transverse modes above the line where it supports longitudinal modes only. Important thermodynamic implications of this finding are discussed. We directly detect positive sound dispersion (PSD) below the Frenkel line where transverse modes are operative, quantitatively explain its magnitude on the basis of transverse and longitudinal velocities. PSD disappears above the Frenkel line which therefore demarcates the supercritical phase diagram into two areas where PSD does and does not operate.