Abnormal behavior of sound velocity and damping in the vicinity of the smectic-A-smectic-C transition in terephthal-bis-p-p'-butylaniline (TBBA).

Abstract

Sound velocity and damping measurements, taken at various angles $\ensuremath{\theta}$ between the normal to the smectic layers and the sound propagation direction, are reported for TBBA near its $\mathrm{Sm}A\ensuremath{-}\mathrm{S}\mathrm{m}C$ transition. Both velocity and damping present marked pretransitional anisotropic effects above ${T}_{\mathrm{Ac}}$. The anisotropy of the transition also manifests itself on the positions of the damping peaks of which that for $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$ moves towards low temperatures as the frequency increases, whereas no displacement is observed for 0=45\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{}. The results are compared with the predictions of the current theories.