Measurement of Smectic-Phase Order-Parameter Fluctuations in the Nematic Phase of Heptyloxyazoxybenzene

A measurement is presented of the liquid-structure factor of $p-n$-heptyloxyazoxybenzene in the nematic phase using monochromatic Cu $K\alpha$ radiation. This material has a smectic $C$ phase at lower temperature and exhibits strong pretransition scattering in the nematic phase. Near the phase transition the liquid-structure factor peaks at a tilt angle of 30° between the plane normal and the nematic director. By using a simple Landau-like theory to describe the phenomenon, one can map out the free energy of a smectic $C$ density wave as a function of wavelength, tilt angle, and temperature.