Nature of the smectic-A—smectic-Cphase transition in liquid crystals

Abstract

Experimentally we present high-resolution ac calorimetry measurements on a liquid-crystal compound exhibiting a smectic-$A$—smectic-$C$ ($\mathrm{Sm}A-\mathrm{Sm}C$) phase transition which is shown to be mean-field-like. In the light of this experimental result, a simple and convincing argument, based on the mean-field theory, is given to explain why non-mean-field critical exponents were obtained in many of the studies on the $\mathrm{Sm}A-\mathrm{Sm}C$ transition. The theory not only gives an excellent fit to our data for temperatures below the transition temperature but also provides a qualitative prediction on the size of the deviation from the linear relationship between ($T_c-T$) and the square of the order parameter. The predicted deviation is consistent with x-ray measurements on the order parameter of another liquid-crystal compound.