Phase transitions with coupled order parameters
A broad set of phenomena in which the coupling of several order parameters is essential is examined from a unitary standpoint on the basis of the fluctuational theory of phase transitions. The coupling of an order parameter with other degrees of freedom can lead to the appearance on the phase transition lines of singular (polycritical) points, near which the character of the anomalies changes. We analyze numerous experimental studies near polycritical points, whose results as a whole confirm the conclusions of fluctuational theory. The studied objects include $^3$He --$^4$He solutions, antiferromagnetics, ammonium halides, liquid crystals, and mercury near the liquid-gas critical point. The main result of studying systems with coupled order parameters is a substantial expansion of the possibility of a universal description of real phase transitions, including those that have been objects of more or less separate fields of physics, owing to their complexity.