Unusual melting behavior in a smectic liquid crystal

Sound-velocity and attenuation measurements are reported for the smectic liquid crystal $p$-butoxybenzylidene-$p^{\prime }$-octylaniline. At the smectic-$A$ smectic-$B$ transition temperature $T_{\mathrm{AB}}$, $v$ and $\alpha$ change rapidly, with a peak in $\alpha$ occurring below $T_{\mathrm{AB}}$. A very small anomaly is also visible 2 to 2.5 degrees above $T_{\mathrm{AB}}$. The data at $T_{\mathrm{AB}}$ are analyzed in terms of dislocation-mediated melting theories by assuming that the transition occurs at a crossover between two- and three-dimensional behavior. The low-frequency velocity change of $\frac{\Delta v}{v}=0.047\mathrm{}$ within the planes agrees well with the predicted value of 0.040.