SmA-SmC Transition in Thin Films

The behavior of a system close to a critical point or to a second order phase transition depends on the spatial dimensionality. Smectic liquid crystals can easily be formed into thin films freely suspended on a frame, and are thus good candidates for investigating the thermodynamic behavior in a reduced dimensionality. As for the phase transitions concerned, on the one hand, the role of fluctuations is expected to become more and more important and to lead, therefore, to a destabilization of the more ordered 

More recently, Bahr and Fliegner studied the behavior of a first order SmA-SmC transition in free-standing liquid crystal films [111]. For films thicker than 15 layers, a first order transition is observed at the same temperature as in the bulk, but the surface layers are always tilted in the whole temperature range, regardless of the film thickness. In films thinner than 15 layers, surface interactions become predominant, and the first 

The typical hexatic in-plane order is characterized by a sixfold modulation of the X-ray diffuse scattering ring corresponding to the molecular correlations. The angular dependence of the X-ray scattering intensity in the plane of the smectic layers can be written as 

In that respect, the SmB ex-SmA transition could be interpreted as belonging to the superfluid helium universality class [114], 

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