Crossover from XY critical to tricritical behavior of heat capacity at the smectic-A-chiral-smectic-C liquid-crystal transition

High-resolution ac calorimetric measurements have been carried out for four chiral liquid-crystal systems: 4-(1-methylheptyloxycarbonyl) phenyl 4'-octyloxybiphenyl-4-carboxylate, its octylbiphenyl analog, its octyloxycarbonylbiphenyl analog, and 2-fluoro-4-{[(1-trifluoromethyl) undecyloxyl carbonyl) phenyl 4'-(dodecyloxy) biphenyl-4-carboxylate. The heat capacity anomaly around the smectic-A to the chiral-smectic-C transition has been analyzed in detail. It is revealed that the heat anomaly shows a crossover from three-dimensional XY critical to tricritical behavior. All the data are described well with a crossover function which has been obtained by a Rudnick-Nelson type calculation.