ORDERING EFFECTS IN THIN SMECTIC-C-ASTERISK FILMS - AN X-RAY-REFLECTIVITY STUDY

The x-ray-reflectivity technique is used to study the smectic order an d the smectic-A (Sm-A) to smectic-C (Sm-C*) phase transition in thin and ultrathin films (150-600 angstrom) of the chiral ferroelectric liq uid-crystal mixture ZLI-3654 (produced by Merck), The films, which are spin cast on various substrates [very smooth (float) glass, Si Wafer, polymer-coated glass, etc.], order spontaneously with smectic layerin g parallel to the substrate surface; the film alignment is induced by anchoring forces at the film-air interface. The reflectivity profiles could be well described by a sinusoidal density modulation perpendicul ar to the film. We demonstrate that it is possible to extract the mole cular tilt angle alpha in ferroelectric liquid crystals from x-ray-ref lectivity measurements of thin films. The Sm- A - Sm-C phase-transiti on temperature and the temperature dependence of the tilt angle in the smectic-C phase are almost independent of the film thickness (down t o approximately 200 angstrom) and are similar to those in the bulk. In all cases the dependence of the tilt angle alpha(T) can be described by a power law: alpha approximately t(gamma) '' (t = 1 - T/T(c)), wher e gamma = 0.31 +/- 0.04, in agreement with the de Gennes predicted sup erfluid helium analogy [Acad. Sci. Paris Ser. B 274, 785 (1972)]. In a film of about 200-angstrom thickness we observed a smectic layer spac ing which is much larger than in thick films.