THE SMECTIC-C SMECTIC-A TRANSITION IN FREESTANDING LIQUID-CRYSTAL FILMS - AN EXAMPLE FOR CRITICAL-BEHAVIOR INDUCED BY A CONFINED GEOMETRY

We report on an ellipsometric study of a phase transition in thin free-standing films of a smectic liquid crystal. The smectic-C- smectic-A transition, which is first-order in bulk, becomes continuous in films thinner than about 15 molecular layers indicating the influence of a strong ordering field resulting from surface interactions. The experimental results are discussed within the framework of a simple Landau model consisting of the bulk parameters and extended by a linear coupling to a surface field which decays exponentially from the surface to the interior of the film.