Magnetic-susceptibility measurements below a nearly-second-order nematic-isotropic phase transition in a lyotropic liquid crystal

Magnetic-birefringence measurements were performed in the nematic phase of the binary lyotropic liquid crystal cesium perfluoro-octanoate and water within 75 mK of the superheating limit. Several results were obtained: (1) The induced birefringence was linear in H for small H, but showed significant deviations from linearity for H>5 T; (2) the elastic constant was found to be very small near the transition, indicating that director fluctuations may contribute a significant multiplicative factor to the average order parameter; and (3) the order-parameter exponent {β}=0.34±{}0.06. This last result is examined in the context of a mean-field theory with higher-order terms in the free-energy expansion and in the context of the d=3, N=5 N-vector model.