Thermally triggered phononic gaps in liquids at THz scale

TL;DR

This study combines experiments and simulations to reveal how phononic gaps in liquid argon emerge at THz frequencies with temperature changes, highlighting potential for advanced THz thermal devices.

Contribution

It demonstrates the first observation of strong localization of longitudinal phononic modes and the emergence of transverse phononic gaps in liquids at THz scales, linked to the Frenkel line transition.

Findings

Low-frequency transverse phononic gap appears with temperature increase.

High-frequency propagating modes become evanescent at THz scale.

Localization of longitudinal phononic modes observed in supercritical phase.

Abstract

In this paper we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials.