ANISOTROPIC RAYLEIGH-SCATTERING INVESTIGATIONS IN SMECTIC A LIQUID-CRYSTAL PHASES

We present here a complete study of Rayleigh scattering by the smectic A phases of 8CB and 80CB (octylcyanobiphenyl and octyloxycyanobiphenyl). Using two original experimental setups we have been able, for the first time, to measure both the scattered intensities due to orientational fluctuations appearing in a plane containing the mean optical axis of the phase and the ones, more original, in the plane of the smectic layers. We show that these ''in plane'' fluctuations are unexpectedly important and should be taken into account in the future. We also demonstrate-from both static and dynamic data-that these fluctuations are due to orientational fluctuations of the optical axis and calculate the associated Rayleigh intensities. The confrontation between the calculated and the experimental values allows the determination of both the order of magnitude and the thermal behavior of new elastic constants. Moreover, our results determine the limit of the domain of the so-called linear approximation frequently used in the past in the physics of liquid crystals.