Using nematic director fluctuations as a sensitive probe of the nematic-smectic-A phase transition in liquid crystals.

We present a new, extremely sensitive optical method to study the nematic-smectic-A (NA) phase transition in liquid crystals. In this new technique, we monitor nematic director fluctuations as a function of temperature as we approach the NA transition. We show that the NA transition in the liquid crystal 8CB (which has a small nematic range) is clearly first order, well within the resolution of the experiment. This is contrary to previous calorimetric measurements but confirms an earlier dynamical measurement. We characterize the strength of the phase transition by a dimensionless, technique-independent quantity t(0) = (T-NA - T*)/T-NA. For 8CB, we measure t(0) = (6 +/- 2) x 10(-6).