Critical behavior at nematic–smectic-Aphase transitions

The effective experimental critical exponents {α}, {γ}, ν_∥, and ν_⊥ obtained from high-resolution heat capacity and x-ray studies of nematic--smectic-A transitions in liquid crystals are presented as a function of the ratio TNA/TNI of the nematic--smetic-A transition temperature to that of the nematic-isotropic transition. These results, which are the most extensive ever reviewed, show complex systematic trends that are compared with the theoretically predicted crossover from three-dimensional XY to tricritical behavior and the anisotropic behavior predicted to arise due to coupling between the smectic order parameter and director fluctuations.