Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of the fluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter, Ψ₆, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find that Ψ₆ is two-dimensional whereas the positional order in the crystalline S_J phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensional XY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.