Fluctuations of the Order Parameter in Phenomenological Theory

We will now be concerned with the assumptions that were made in the construction of the Landau phenomenological theory of phase transitions. The basic assumption of the theory is the assumption of the normal, “nonsingular” temperature dependence of the coefficients in the expansion of the incomplete themodynamic potential. It was assumed, for example, that the dependence Φ₀(p,T) in the vicinity of the phase transition point is the same as it is far away from it. By definition, the incomplete thermodynamic potential Φ₀(p,T) takes account of the contribution of all degrees of freedom, except η, to the complete thermodynamic potential. It was assumed implicitly that these degrees of freedom “do not feel” a phase transition. The concept that the phase transition does not “touch upon” other degrees of freedom, except η, lies also implicitly at the basis of the assumption of the “nonsingular” temperature dependence of the coefficients in the expansion of the thermodynamic potential.