Supercritical behavior of magnetic and liquid model systems

Abstract

The supercritical transitions are widely occurring. They include the supercritical transitions in the liquid-vapor system, ferromagnetic transitions, transitions in polymers, many transitions in liquid crystals, and some structural transitions. In the paper it is emphasized that the nature of the critical and supercritical transitions is the same – these are continuous fluctuation transitions. Above the critical temperature the system passes through a region of lowered stability, which leads to increase of fluctuations of energy and external parameters of the system. From the point of view of thermodynamic stability this indicates the existence of a continuous supercritical transition between supercritical mesophases. Knowing the basic stability characteristics of a system, we derive the equation of these mesophase transitions. Depending on a thermal equation type, we can get one or several such equations, which may not coincide. This approves the fact that a supercritical transition occurs in a certain interval of thermodynamic forces.
 In the paper the relations between the critical exponents of thermodynamic parameters of the system are obtained and the conditions of continuous conjugation of the lowered stability line to subcritical coexistence line are investigated. The results are applied to the Curie–Weiss and van der Waals models: we obtain the quasi-spinodal equation for these systems and analyze the critical and supercritical behavior of the stability characteristics.