Ultrasonic Observation of a Strong Pretransitional Anomaly near a Nematic-Smectic-APhase Transition

Strong anomalies in the velocity and attenuation of longitudinal ultrasound in the megahertz range were observed for the first time in the vicinity of the nematic-smectic-$A$ phase transition in a liquid crystal (terephthal-bis-$p-p^{\prime }$-butylaniline). Results are in strong contradiction with theoretical predictions and are qualitatively consistent with a picture involving both critical fluctuations and relaxation of the smectic order parameter, similar to that observed near the $\lambda$ transition in liquid helium.