Optical and X-Ray Studies of Critical Phenomena in Thin Liquid Crystal Films.

Structural phase transitions in thin, freely-suspended films of liquid crystals were studied using ellipsometry, light-scattering and x-ray scattering techniques. The smectic-C tilt order parameter was found to obey a scaling relation as a function of temperature and film thickness indicating that only the surface layers order over a wide range of temperatures. Deep within the smectic-C phase, the tilt order parameters for all thickness films measured converged upon a unique value at which the tilted fluid to tilted hexatic smectic-C to smectic-I transition took place. The light-scattering intensities and decay times above the smectic-C to smectic-A transition, which resembles the two-dimensional XY model in thin films, were found to scale with the Kosterlitz-Thouless correlation length, as predicted by dynamical theories of the transition. Measurements of the predicted Nelson-Kosterlitz universal jump in theta concurred with estimates for the bulk material parameters and with the light-scattering results. X -ray scattering results showed that for one material studied, the smectic-I to smectic-C transition takes place by two first-order transitions, the first a surface hexatic ordering transition followed by a surface to bulk smectic-I transition as the temperature is lowered further. Significant pretransitional ordering takes place below the surface smectic-I to smectic -C transition, while the surface to bulk smectic-I transition takes place by one abrupt step, with no layer-by-layer growth of the hexatic order into the film.