Quasicritical heat capacity at a smectic-A –hexatic-B phase transition

High-resolution ac calorimetry has been used to characterize the excess heat capacity DCp(T) associated with the smectic-A (SmA) –hexatic-B (HexB) transition in n-hexyl-48-n-pentyloxybiphenyl-4-carboxylate ~65OBC!. For temperature oscillations in the investigated frequency range 1.42–400 mHz, the DCp data reveal no frequency dependence and correspond to static thermodynamic values. The present data resolve several discrepancies in previously reported Cp values. This transition is very weakly first order, but power-law fits could be made to DCp . Such fits yield an effective critical exponent aeff50.6560.05, which agrees quite well with values of ;0.6 reported 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 exponent value disagrees strongly with the theoretically predicted three-dimensional XY value aXY520.007. It is proposed that this Cp behavior and also the reported order parameter variation, which yields an effective critical exponent beff of ;0.2, are consistent with quasitricritical or quasitetracritical ~strain-smeared, very weak first-order! behavior arising from a coupling between the amplitude of the bond-orientational order and the in-plane positional strain. @S1063-651X~97!05908-4#