Fluctuations above the Smectic-A-Isotropic Transition in Liquid Crystalline Elastomers under External Stress

Using birefringence measurements, we observe for the first time nematic as well as smectic fluctuations above the smectic-A-isotropic transition in liquid crystalline elastomers under an external mechanical stress. From a mean-field analysis we find that there are temperatures (far above the isotropic-A transition) for which nematic fluctuations dominate as well as a range of temperatures (above and close to this phase transition) where smectic fluctuations are more important. These two regimes are connected by a crossover regime where the coupling between the two types of fluctuations becomes important. We also discuss the possibility of a triple point in liquid crystalline elastomers, where an isotropic, a smectic and a nematic phase meet.