INPLANE ORIENTATIONAL FLUCTUATIONS IN SMECTIC-A LIQUID-CRYSTALS

We present here some experimental Rayleigh-scattering investigations o f the fluctuations appearing in the plane of the layers of the smectic -A phase of 8CB (octylcyanobiphenyl) and 8OCB (octyloxycyanobiphenyl). Their origin is identified and is found to be due to the fluctuations of the director of the phase. Their amplitude is measured and compare d to the more classical and previously studied fluctuations appearing in the vertical plane containing the director. Moreover, we present a theoretical calculation of the associated scattered intensity that see ms to be in good agreement with the experimental results up to the vic inity of the phase transition. The consequences of the unexpected rela tive importance of such ''in-plane'' fluctuations are discussed.