Birefringence suppression near the smectic A-smectic C phase transition in de Vries-type liquid crystals

Two smectic liquid crystal materials exhibiting a de Vries-type smectic A to smectic C transition with low layer shrinkage were studied using polarised microscopy. Birefringence and tilt angle was measured as a function of temperature using a novel camera and software system. The analysis showed unusual features in birefringence, which had a peak within the smectic A phase and decreased significantly with cooling for a similar to 15 degrees C range above the smectic A-smectic C transition in both materials. The reduction in birefringence above the smectic A-smectic C transition was interpreted with a critical fluctuation model, which allowed calculation of the heat capacity critical exponent a. Exponents were not in a clear universality class, similar to previously reported results on other non-de Vries materials. However, a recently published mean field model for de Vries smectics provided an alternate explanation for birefringence suppression. Five analyses were performed on the birefringence and tilt angle data and were interpreted using new predictions of this mean field model. Our results for the observed reduced temperature range |T-T-C|/T-C > 10(-3), where T is temperature and T-C is the smectic A-smectic C transition temperature, were consistent with all mean field theory predictions for a transition near or at a tricritical point.