Phenomenological theory of the smectic A-nematic transition

We start with the density wave in the smectic phase 

Near the transition, the free energy density may be expanded in powers of ly as follows 

From symmetry considerations it is clear that only even powers of y/ may be included. The coefficient P is always positive. At a certain temperature T, which is the second order transition point, a = 0. Accordingly, as explained in 2.5.1, we set in the mean field approximation, 

To allow for the coupling between y/ and the orientational order parameter s, the free energy is expressed as  

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