Measurement of Smectic-A- Phase Order-Parameter Fluctuations in the Nematic Phase ofp−n-Octyloxybenzylidene-p′-Toluidine

A measurement is presented of the anisotropic liquid-structure factor of $p-n$-octyloxybenzylidene-$p^{\prime }$-toluidine in the nematic phase using monochromatic Cu $K\alpha$ radiation. This material has a smectic $A$ phase at lower temperature and exhibits strong pretransition scattering in the nematic phase. The experimental results are used to test the Landau theory of the smectic $A$ phase recently proposed by McMillan and by de Gennes. The experimental peak in the structure factor is approximately Lorentizian in two dimensions and a Lorentzian fit determines the Landau-theory parameters versus temperature. The small deviations from a Lorentzian may be due to higher-order terms in the Landau theory or to thermal fluctuations of the nematic director.