CRITICAL-BEHAVIOR AT NEMATIC SMECTIC-A PHASE-TRANSITIONS

The effective experimental critical exponents alpha, gamma, nu(paralle l-to), and nu(perpendicular-to) obtained from high-resolution heat cap acity and x-ray studies of nematic-smectic-A transitions in liquid cry stals are presented as a function of the ratio T(NA)/T(NI) of the nema tic-smetic-A transition temperature to that of the nematic-isotropic t ransition. These results, which are the most extensive ever reviewed, show complex systematic trends that are compared with the theoreticall y predicted crossover from three-dimensional XY to tricritical behavio r and the anisotropic behavior predicted to arise due to coupling betw een the smectic order parameter and director fluctuations.