Evidence for structural crossover in the supercritical state

TL;DR

This paper reveals a well-defined structural crossover in the supercritical state, challenging the notion of uniformity above the critical point and linking structure with dynamics and thermodynamics.

Contribution

It demonstrates the existence of a structural crossover in the supercritical state, showing qualitative changes in atomic arrangements and correlations.

Findings

Identification of a structural crossover separating liquid-like and gas-like states.

Correlation between structural, dynamic, and thermodynamic crossovers.

Evidence of medium-range structural correlations in the supercritical state.

Abstract

The state of matter above the critical point is terra incognita, and is loosely discussed as a physically homogeneous flowing state where no differences can be made between a liquid and a gas and where properties undergo no marked or distinct changes with pressure and temperature. In particular, the structure of supercritical state is currently viewed to be the same everywhere on the phase diagram, and to change only gradually and in a featureless way while moving along any temperature and pressure path above the critical point. Here, we demonstrate that this is not the case, but that there is a well-defined structural crossover instead. Evidenced by the qualitative changes of distribution functions of interatomic distances and angles, the crossover demarcates liquid-like and gas-like configurations and the presence of medium-range structural correlations. Importantly, the discovered structural crossover is closely related to both dynamic and thermodynamic crossovers operating in the supercritical state, providing new unexpected fundamental interlinks between the supercritical structure, dynamics and thermodynamics.