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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.
Highlights
An increase in temperature leads to the disappearance of both the high-frequency transverse phononic modes and progressively the medium-range pair correlations, which we show in detail
The presented experimental and MD simulations results are explained within the framework of unified phonon-based approach[4] where the problem of strong interactions is tackled from the outset (see Eq (1))
The intra pair distribution function g(r) exhibits linear dependence on pressure variation making it impossible to detect any thermodynamic boundaries in the supercritical phase[26]
Summary
Our results show that high-frequency phononic gaps (both transverse and longitudinal) appear on picosecond time scale at elevated temperatures. Tuning of the atomic dynamics and thermally triggered localization of low- and high-frequency propagating phononic modes are the key factors for the advancement of technologies based on the sound control and manipulation at the THz regime. Viscosity η(T) drops down upon temperature rise resulting in the transverse phononic gap growth due to the Frenkel frequency ωF (see Eq (2)) increase.
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