# The nature of collective excitations and their crossover at extreme supercritical conditions

## TL;DR

This study provides direct evidence of solid-like collective excitations in supercritical fluids at extremely high temperatures, revealing a crossover from phonon-like to collisional dynamics that scales with temperature.

## Contribution

It is the first to observe and characterize propagating phonon-like excitations deep in the supercritical state at unprecedented temperatures.

## Key findings

- Propagation of solid-like phonon excitations at high temperatures
- Crossover from collective to collisional regime occurs at specific wavelengths
- Crossover points scale with temperature following the same power law

## Abstract

Physical properties of an interacting system are governed by collective excitations, but their nature at extreme supercritical conditions is unknown. Here, we present direct evidence for propagating solid-like longitudinal phonon-like excitations with wavelengths extending to interatomic separations deep in the supercritical state at temperatures up to 3,300 times the critical temperature. We observe that the crossover of dispersion curves develops at $k$ points reducing with temperature. We interpret this effect as the crossover from the collective phonon to the collisional mean-free path regime of particle dynamics and find that the crossover points are close to both the inverse of the shortest available wavelength in the system and to the particle mean free path inferred from experiments and theory. Notably, both the shortest wavelength and mean free path scale with temperature with the same power law, lending further support to our findings.