Basic principles of phenomenological theory for second order phase transitions (PT) were formulated
by Landau in late 30–40s [1, 2], originally to describe superconductivity. The key assumption
of the theory is that in the vicinity of the critical point, the free energy can be expanded in a
power series in the order parameter, the equilibrium value of which is found to obey the minimum
condition for free energy. As is now known [3], this expansion is well defined in the neighborhood
of Tc (critical value of temperature) except for a narrow interval determined by the Ginzburg
criterion [4]. In this interval, the crucial role is played by the order parameter fluctuations while
Landau’s theory is essentially a mean-field theory. Nevertheless, it is of great use as a qualitative
tool for understanding the nature of PT