Critical behavior at nematic–smectic-Aphase transitions

The effective experimental critical exponents α, γ, ${\nu }_{\mathrm{\parallel }}$, and ${\nu }_{\mathrm{\perp }}$ 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 ${\mathit{T}}_{\mathit{N}\mathit{A}}$/${\mathit{T}}_{\mathit{N}\mathit{I}}$ 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.