Two liquid states of matter: A new dynamic line on a phase diagram

Abstract

It is generally agreed that the supercritical region of a liquid consists of one single state (supercritical fluid). On the other hand, we show here that liquids in this region exist in two qualitatively different states: "rigid" and "non-rigid" liquid. Rigid to non-rigid transition corresponds to the condition {\tau} ~ {\tau}0, where {\tau}is liquid relaxation time and {\tau}0 is the minimal period of transverse quasi-harmonic waves. This condition defines a new dynamic line on the phase diagram, and corresponds to the loss of shear stiffness of a liquid at all available frequencies, and consequently to the qualitative change of many important liquid properties. We analyze the dynamic line theoretically as well as in real and model liquids, and show that the transition corresponds to the disappearance of high-frequency sound, qualitative changes of diffusion and viscous flow, increase of particle thermal speed to half of the speed of sound and reduction of the constant volume specific heat to 2kB per particle. In contrast to the Widom line that exists near the critical point only, the new dynamic line is universal: it separates two liquid states at arbitrarily high pressure and temperature, and exists in systems where liquid - gas transition and the critical point are absent overall.