Quasicritical heat capacity at a smectic-A–hexatic-Bphase transition

Abstract

High-resolution ac calorimetry has been used to characterize the excess heat capacity $\ensuremath{\Delta}{C}_{p}(T)$ associated with the smectic-$A$ $(\mathrm{Sm}A)$ --hexatic-$B$ $(\mathrm{Hex}B)$ transition in $n$-hexyl-${4}^{\ensuremath{'}}\ensuremath{-}n$-pentyloxybiphenyl-4-carboxylate (65OBC). For temperature oscillations in the investigated frequency range 1.42--400 mHz, the $\ensuremath{\Delta}{C}_{p}$ data reveal no frequency dependence and correspond to static thermodynamic values. The present data resolve several discrepancies in previously reported ${C}_{p}$ values. This transition is very weakly first order, but power-law fits could be made to $\ensuremath{\Delta}{C}_{p}.$ Such fits yield an effective critical exponent ${\ensuremath{\alpha}}_{\mathrm{eff}}=0.65\ifmmode\pm\else\textpm\fi{}0.05,$ which agrees quite well with values of $\ensuremath{\sim}0.6$ 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 $\mathrm{XY}$ value ${\ensuremath{\alpha}}_{\mathrm{XY}}=\ensuremath{-}0.007.$ It is proposed that this ${C}_{p}$ behavior and also the reported order parameter variation, which yields an effective critical exponent ${\ensuremath{\beta}}_{\mathrm{eff}}$ of $\ensuremath{\sim}0.2,$ 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.