High-resolution ac calorimetric, x-ray, and light-scattering measurements have been made on N-[4-(n-butyloxy)benzylidene]-4'-(n-heptyl)aniline (4O.7) near the nematic-smectic-A phase transition. This transition is second order, with a variation in Cp which is well described by a critical exponent α=α'=-0.026 corresponding to the three-dimensional XY model. The longitudinal and transverse correlation lengths exhibit single-power-law divergences over the reduced-temperature range 10-2 to 3×10-5 with exponents ν∥=0.78+/-0.02 and ν⊥=0.65+/-0.02, respectively. The anisotropic hyperscaling relation ν∥+2ν⊥+α=2 is satisfied to within the errors. A dynamical-scaling analysis of light-scattering measurements of the bend-mode director fluctuations with elastic constant K3 yields values for ξ∥ which agree absolutely with the x-ray measurements. Thus the data are all internally consistent, and they appear to exclude all current theories of the nematic-smectic-A transition. It is observed that the overall critical behavior in smectic liquid crystals is sensitive to the width of the nematic range; this may offer an explanation for the apparent nonuniversality of the exponents.
Physical Review A (General Physics) 27(6), pp.3234