We discuss the fluid state of matter at high temperature and pressure. We review the existing ways in which the boundary between a liquid and a quasigas fluid above the critical point are discussed. We show that the proposed ’thermodynamic’ continuation of the boiling line, the ’Widom line’, exists as a line near the critical point only, but becomes a bunch of short lines at a higher temperature. We subsequently propose a new ’dynamic’ line separating a liquid and a gas-like fluid. The dynamic line is related to different types of particle trajectories and different diffusion mechanisms in liquids and dense gases. The location of the line on the phase diagram is determined by the equality of the liquid relaxation time and the minimal period of transverse acoustic excitations. Crossing the line results in the disappearance of transverse waves at all frequencies, the diffusion coefficient acquiring a value close to that at the critical point, the speed of sound becoming twice the particle thermal speed, and the specific heat reaching 2k_B. In the high-pressure limit, the temperature on the dynamic line depends on pressure in the same way as does the melting temperature. In contrast to the Widom line, the proposed dynamic line separates liquid and gas-like fluids above the critical point at arbitrarily high pressure and temperature. We propose calling the new dynamic line the ’Frenkel line’.

PACS: 62.10.+s, 62.50.−p, 63.50.−x, 64.60.F−, 64.60.fd, 65.20.De, 66.20.Cy (all) DOI: 10.3367/UFNe.0182.201211a.1137 URL: https://ufn.ru/en/articles/2012/11/a/ https://youtu.be/z4uAHLiUSmo https://rutube.ru/video/87a452b31ab611e02222429ddb883d29/ 000314808600001 2-s2.0-84873901364 2012PhyU...55.1061B Citation: Brazhkin V V, Lyapin A G, Ryzhov V N, Trachenko K, Fomin Yu D, Tsiok E N "Where is the supercritical fluid on the phase diagram?" Phys. Usp. 55 1061–1079 (2012) BibTexBibNote ® (generic)BibNote ® (RIS)MedlineRefWorks Received: 20th, September 2011, revised: 31st, October 2011, accepted: 2nd, November 2011 Оригинал: Бражкин В В, Ляпин А Г, Рыжов В Н, Траченко К, Фомин Ю Д, Циок Е Н «Где находится область сверхкритического флюида на фазовой диаграмме?» УФН 182 1137–1156 (2012); DOI: 10.3367/UFNr.0182.201211a.1137