Dynamic scaling of ultrasonic damping near the nematic-smectic-A transition of TBBA

We report a detailed study of the ultrasonic damping near the nematic-smect ic-A phase transition of terephtal-bis-p-p'-butylaniline (TBBA). Two mechan isms contribute to the damping. One is isotropic: it is associated with the critical fluctuations, and leads to a scaling behavior of the 3D-XY type w hich is observed up to 15 degrees C from the transition, even though the tr ansition is first order. The other is anisotropic: it appears only in the s mectic-A phase, and is associated with the relaxation of the order-paramete r modulus.