Brillouin scattering in the isotropic, nematic, and smectic-Aphases ofp-cyanobenzylidene-p-n-octyloxylaniline

The results of Brillouin scattering from $p$-cyanobenzylidene-$p$-$n$-octyloxylaniline are reported for the isotropic liquid phase and for monodomains of the nematic and smectic-$A$ phases. For the isotropic phase, the observed temperature dependences of the velocity and the attenuation of sound can be explained well by the temperature dependences of the density, compressibility, and viscosity. In the two liquid-crystalline phases, the velocities of sound are found to depend only weakly on the propagation direction relative to the orientations of the long molecular axes and to vary smoothly with temperature, without irregularities at the nematic-smectic-$A$ transition. The measurements do not reveal the propagating undulation mode (second sound) predicted for the smectic-$A$ phase. The observed directional dependence of the first-sound propagation suggests, however, that second sound exists, but with velocities which are too low for detection in the present experiment. For the isotropic phase, separate measurements were performed on the intensities and widths of polarized and depolarized Rayleigh scattering. These results agree with the predictions of the Landau-de Gennes model. Both the order-parameter relaxation time and the integrated intensities of critical scattering diverge towards the isotropic-nematic transition with a ${(T-T_c^{*})}^{-1\mathrm{}}$ temperature dependence.