The birefringence $\Delta n,$ the specific heat $C_p,$ and the layer compressional elastic modulus B are reported for two liquid crystals near the nematic $\mathrm{}\left(N\right)\mathrm{}$ to smectic-$A$ $\left(\mathrm{Sm}A\right)$ phase transition. As predicted long ago by MacMillan and de Gennes [P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Clarendon, Oxford, 1993)] the coupling of the nematic orientational order parameter to the smectic-$A$ layering order parameter can substantially alter the critical behavior near the $N-\mathrm{Sm}A$ transition if the nematic range is small and the nematic order parameter susceptibility is large. In this paper, we present a direct experimental comparison of two compounds: 4-octyloxy-$4^{\prime }$-cyanobiphenyl (8OCB) with a short nematic range and 4-octyloxybenzoyloxy-$4^{\prime }$-cyanotolane $\left({\mathrm{C}}_8\mathrm{tolane}\right)$ with a very large $N$ range. The temperature variations of the apparent birefringence $\Delta n$ and the specific heat $C_p$ across the N-Sm$A$ phase transition show the definite influence of the proximity of the isotropic phase in the case of 8OCB while the ${\mathrm{C}}_8\mathrm{tolane}$ behaves as expected for the three-dimensional $\mathrm{XY}$ universality class. The elastic modulus B in the Sm$A$ phase, measured at several wave vectors by the second-sound resonance technique, was studied with high resolution as a function of temperature on approaching $T_c(N-\mathrm{Sm}A).\mathrm{}$ These elastic data confirm the B leveling off in both cases with an apparent breakdown of hydrodynamics in the case of the ${\mathrm{C}}_8\mathrm{tolane}$ compound.

• Received 24 March 1997

DOI:https://doi.org/10.1103/PhysRevE.56.5566