Critical behavior at nematic–smectic-<i>A</i>phase transitions

Abstract

The effective experimental critical exponents \ensuremath{\alpha}, \ensuremath{\gamma}, ${\ensuremath{\nu}}_{\mathrm{\ensuremath{\parallel}}}$, and ${\ensuremath{\nu}}_{\mathrm{\ensuremath{\perp}}}$ 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.