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

The orientational order parameter decrease, due to nematic director macrosc opic fluctuations, is obtained by birefringence measurements around the nem atic <----> smectic-A transition temperature of the liquid crystal 4'-n-oct yl-4-cyanobiphenyl. The measured nematic order reduction shows the same amp litude as the secondary order parameter discontinuity at the transition pro viding direct evidence of the Halperin-Lubensky-Ma effect. The importance o f nematic director thermal fluctuations on the character of the transition is revealed as their quenching by an electric field of similar to 20 V/mum restores the second order character of the transition.