3D XY BEHAVIOR OF A NEMATIC-SMECTIC-A PHASE-TRANSITION - CONFIRMATION OF THE DEGENNES MODEL

A high-resolution x-ray study of the nematic-to-smectic-A1 phase transition of 8OPCBOB gives a critical exponent for the susceptibility gamma = 1.39 +/- 0.08, and for the parallel correlation length nu parallel-to = 0.71 +/- 0.04. These values, as well as the known exponent of the heat capacity, agree with the predictions from the 3D XY model. The different critical behavior of the perpendicular correlation length (nu perpendicular-to = 0.56 +/- 0.05) is shown to be related to the quartic term in q perpendicular-to present in the structure factor. Our analysis resolves a long-standing puzzle about the critical behavior of this phase transition.