Experimental evidence of the Halperin-Lubensky-Ma effect in liquid crystals.

The orientational order parameter decrease, due to nematic director macroscopic fluctuations, is obtained by birefringence measurements around the nematic <--> smectic-A transition temperature of the liquid crystal 4'-n-octyl-4-cyanobiphenyl. The measured nematic order reduction shows the same amplitude as the secondary order parameter discontinuity at the transition providing direct evidence of the Halperin-Lubensky-Ma effect. The importance of nematic director thermal fluctuations on the character of the transition is revealed as their quenching by an electric field of approximately 20 V/micrometer restores the second order character of the transition.