QUASICRITICAL HEAT-CAPACITY AT A SMECTIC-A-HEXATIC-B PHASE-TRANSITION

High-resolution ac calorimetry has been used to characterize the exces s heat capacity Delta C-p(T) associated with the smectic-A (SmA) -hexa tic-B (HexB) transition in n-hexyl-4'-n-pentyloxybiphenyl-4-carboxylat e (650BC). For temperature oscillations in the investigated frequency range 1.42-400 mHz, the Delta C-p data reveal no frequency dependence and correspond to static thermodynamic values. The: present data resol ve several discrepancies in previously reported C-p values. This trans ition is very weakly first order, but power-law fits could be made to Delta C-p. Such fits yield an effective critical exponent alpha(eff)=0 .65+/-0.05, which agrees quite well with values of similar to 0.6 repo rted by Huang and co-workers [Phys. Rev. Lett. 46, 1289 (1981); Phys. Rev., A 28, 2433 (1983): Phys. Rev. Lett. 56, 1712 (1986)]. Such an ex ponent value disagrees strongly with the the thoretically predicted th ree-dimensional XY value alpha(XY)=-0.007. It is proposed that this C- p behavior and also the reported order parameter variation, which yiel ds an effective critical exponent beta(eff) of similar to 0.2, are con sistent with quasitricritical or quasitetracritical (strain-smeared, v ery weak first-order) behavior arising from a coupling between the amp litude of the bond-orientational order and the in-plane positional str ain.