High-Resolution Heat-Capacity Studies near the Nematic-Smectic- A Transition 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. α⁺=α^- is near zero on one sample, agreeing with Johnson's measurements, while α⁺=α^- 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.