Unusual melting behavior in a smectic liquid crystal

Sound-velocity and attenuation measurements are reported for the smectic liquid crystal p-butoxybenzylidene-p′-octylaniline. At the smectic-A smectic-B transition temperature TAB, v and α change rapidly, with a peak in α occurring below TAB. A very small anomaly is also visible 2 to 2.5 degrees above TAB. The data at TAB 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 Δvv=0.047 within the planes agrees well with the predicted value of 0.040.