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

Abstract

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 ˜15°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 α. 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 , 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.

Keywords:

• smectic
• de Vries
• SmA–SmC phase transition
• critical behaviour

Acknowledgements

We give great thanks to conversations with Karl Saunders concerning theoretical interpretation of our data and Matt Moelter for his advice on the clarity of presentation. Thanks to Jawad Naciri and Noel Clark for providing materials used in these studies. This research was supported by the Office of Naval Research, Award No. N00014-07-1-1152.