High-Resolution Heat-Capacity Studies near the Nematic-Smectic-ATransition in Octyloxycyanobiphenyl (8OCB)

High-resolution ac calorimetry measurements on two different samples of 8OCB whose transition temperatures differ by less than 0.1 K show different critical behavior at the nematic-smectic-$A$ transition. ${\alpha }^{+}={\alpha }^{-}$ is near zero on one sample, agreeing with Johnson's measurements, while ${\alpha }^{+}={\alpha }^{-}$ equals 0.25 on the second sample which agrees with Garland's measurements on 8OCB. These results suggest the possibility that the nematic-smectic-$A$ transition on 8OCB is near a tricritical point or crossover between two types of critical behavior.