Test of clock model in ellipsometric study of thin and thick free-standing films of an antiferroelectric liquid crystal

Abstract

The temperature dependences of the ellipsometric parameters in a weak dc external field are studied in thin and thick free-standing films of MHPOBC. The results for thin films consisting of two, three, and four layers are analyzed within the discrete phenomenological model. We find very good quantitative agreement between the theory and experiment, which indicates an odd-even effect. We find that the XY structures are stable for an odd number of layers, whereas planar, Ising-like structures are stable for an even number of layers. The experiments on thick (several tens of layers) films show a combination of bulklike and free-surface behavior. This is most pronounced at high temperatures, where the interior of the film is not tilted, whereas the layers at the air interfaces show qualitatively similar temperature dependance of the ellipsometric parameters as in the four-layer film.