Abnormal sound damping in the smectic-Cphase of terephthal-bis-p-p’-butylaniline (TBBA): Evidence for anharmonic effects

We present measurements of ultrasonic damping and velocity, carried out in the smectic-C phase of terephthal-bis-p-${\mathrm{p}}^1$-butylaniline (TBBA), showing that conventional hydrodynamics does not apply in the smectic-C phase of liquid crystals. The angular variation of the abnormal part of the damping is quite different from that obtained in the smectic-A phase of the same compound, and can be explained by the fact that the B/C ratio of the elastic constants, which is greater than 1 in the smectic-A phase, becomes less than 1 in the smectic-C phase.