Supercritical fluids as a distinct state of matter characterized by sub-short-range structural order

Abstract: A supercritical fluid (SCF)--the state of matter at temperatures and pressures above the critical point--exhibits properties intermediate between those of a liquid and a gas. However, whether it constitutes a fundamentally distinct phase or merely a continuous extension of the liquid and gas states remains an open question. Here we show that a SCF is defined by sub-short-range (SSR) structural order in the spatial arrangement of particles, setting it apart from the gas (disordered), liquid (short-range ordered), and solid (long-range ordered) states. The SSR structural order can be characterized by a length scale effectively quantified by the number of observable peaks in the radial distribution function. This length grows from a minimum microscopic value, on the order of the inter-particle distance at the gas-SCF boundary, to a diverging value at the SCF-liquid boundary. Based on the emergence of SSR order, we demonstrate that the transport and dynamical properties of the SCF state, including the diffusion coefficient, shear viscosity, and velocity autocorrelation function, also clearly distinguish it from both the liquid and gas states. Theoretical predictions are validated by molecular dynamics simulations of argon and further supported by existing experimental evidence. Our study confirms and assigns physical significance to the refined phase diagram of matter in the supercritical region, consisting of three distinct states (gas, supercritical fluid, and liquid) separated by two crossover boundaries that follow universal scaling laws.