Ultrasonic attenuation in CBOOA near the nematic-smectic-a transition

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Abstract
We report measurements of the attenuation and velocity near the nematic-smectic-A phase transition in p-cyanobenzylidine-p'-octyloxyaniline (CBOOA). These measurements were performed in the frequency range 0.6-25 MHz as a function of the temperature for various orientations of the CBOOA with respect to the ultrasonic propagation. On the nematic side of the transition the ultrasonic absorption presents a critical increase which is more and more apparent as the frequency is decreased and which reflects the dynamics of critical fluctuations of the order parameter. The critical viscosities are deduced from a systematic study of the various anisotropies in attenuation. It is found that the volume viscosities diverge much faster than the shear viscosity and that the enhancement of the volume viscosities is greater than that of the shear viscosity. A dominant contribution from the specific heat may explain the behaviour of the volume viscosities. The critical part of the ultrasonic spectra is analysed in terms of a frequency-dependent specific heat. For the relaxation frequency of the smectic order parameter we find τ -1m∼5 x 108 ΔT/T s -1. This value is consistent with the theoretical prediction and with estimates deduced from light-scattering experiments. The background absorption can be interpreted as being due to two additive contributions : an intramolecular relaxation and the residual part of the critical contribution of the nematic-isotropic transition. In the smectic-A phase the behaviour of the ultrasonic absorption is complicated by the coexistence of several processes. However, by studying the anisotropy in attenuation it is possible to extract a contribution which is tentatively attributed to the relaxation of the order parameter itself. In the temperature range investigated the results are compatible with a transition of mean-field type