Specific heat near the nematic-smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>transition of octyloxycyanobiphenyl

Abstract

High-resolution ac microcalorimetry studies of octyloxycyanobiphenyl, 80CB, reveal a nematic-smetic-$A$ transition consistent with the superfluid analogy as regards the specific-heat exponents, namely $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\lesssim}O(log)$. However, the amplitude ratio, $\frac{A}{{A}^{\ensuremath{'}}}=1.35$, is inconsistent with the superfluid analogy. This may be related to two complicating features of the nematic-smectic-$A$ transition of the 80CB studied. First, the transition appears to be weakly first order and slightly broadened by impurities. The two-phase region is \ensuremath{\sim}0.04 K wide and an analysis of it based on weak-solution theory gives a transition entropy in the range $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{R}_{o}&lt;\ensuremath{\Delta}S&lt;3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{R}_{o}$. Secondly, the specific-heat exponent exhibits sharp crossover above ${T}_{c}$ from $\ensuremath{\alpha}\ensuremath{\gtrsim}0.3$ to $\ensuremath{\alpha}\ensuremath{\sim}O(log)$ as the asymptotic limit is approached.