SmBhex-SmA Transition in Thin Films

The free-standing film is a very interesting physical system, in which the sample thickness can very easily be varied from two to a few hundred molecular layers. It can be used to test the theories of two-dimensional melting, the evolution of phase transitions as a function of the dimensionality of the system, and to investigate substrate-free, two-dimensional transitions and the effect of free surfaces [119]. In order to analyze the small heat capacity anomalies associated with phase transitions in very thin films, a new AC calorimeter technique has been set up [123]. Thus detailed calorimetric studies of free-standing liquid crystal films have been performed near the SmBhex-SmA transition of several compounds [ 124-126]. 

Further experiments on two layer films clearly show a sharp anomaly in the heat capacity behavior, suggesting that the SmBhex-SmA transition is not well described by the theory of defect-mediated 

In conclusion, the physical origin of the surface order in free-standing films is not fully understood. It seems that the smectic C (see Sec. 4.6) as well as the hexatic B smectic phases are stabilized by a free surface with respect to the smectic A phase (see data reported above). X-ray measurements have shown that the layer fluctuations become very small close to the surface of the films [128], and some theoretical arguments [129] are in favor of a quenching of the layer fluctuations, resulting in an enhancement of the hexatic surface order. 

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