  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.
