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

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.

Excitation spectra in fluids: How to analyze them properly

“…1(a,e). The long-standing problem of the calculation of the q -gap has recently attracted considerable interest in view of crossover between liquid- and gas-like collective dynamics in fluids with change in temperature and pressure 8,26,32,34–45,47–58 . Here, we present the calculated excitation spectra near the q -gap and compare the dispersion relations obtained using the two-oscillator model to the results based on the analysis of transverse current spectra.…”

“…Analysis of maxima at given q -values is widely used to obtain dispersion relations in crystals (see, e.g., refs 8,32–36,38–41,52,53,58 ). Such an approach is suitable in this case, since the current spectra C L , T in crystals typically consist of narrow bands on the plane, while the damping rates are negligible and, thus, are assumed to be equal to zero.…”

“…In thermodynamic equilibrium, the velocity distribution has a Maxwellian form and, hence, for the velocity squared (corresponding to free particle dynamics), we havewhere m is the particle mass. Note that the maximum of the distribution (11) at each fixed q value is reached at , where is the most probable velocity 26,58 .…”

“…However, both these approaches have fundamental disadvantages, since anharmonicity effects are taken into account phenomenologically. Recently, excitation spectra in fluids have become of great interest in context of studies concerning the thermodynamics and collective dynamics of fluids 8,26,32–62 , and, in particular, crossover from fluid- to gas-like collective dynamics (Frenkel line) 8,32,34–41,47,48,50–53,56,58 and in the framework of inelastic X-ray or neutron scattering experiments 63–76 . Note that these experiments have already revealed a problem of mixing of longitudinal and transverse modes, which plays an important role in a proper analysis of excitation spectra as will be shown below.…”

Direct Experimental Evidence of Longitudinal and Transverse Mode Hybridization and Anticrossing in Simple Model Fluids

“…Consequently, we believe that detailed analysis of excitations opens up exciting prospects for elaboration of transport phenomena theory and establishing (as yet unknown) relations between individual, collective dynamics, and thermodynamics of fluids. 2,[56][57][58][59][60][61][62][63][64][65] Furthermore, redistribution of the excitation spectra (as result of their anticrossing), can affect the interaction between longitudinal, transverse, and polarization modes in ionic and polar molecular liquids. We leave these interesting problems for future work.…”