INFLUENCE OF NEMATIC RANGE ON BIREFRINGENCE, HEAT-CAPACITY AND ELASTIC-MODULUS NEAR A NEMATIC-SMECTIC-A PHASE-TRANSITION

The birefringence Delta n, the specific heat C-p, and the layer compre ssional elastic modulus B are reported for two liquid crystals near th e nematic (N) to smectic-A (SmA) phase transition. As predicted long a go by MacMillan and de Gennes [P. G. de Gennes and J. Prost, The Physi cs of Liquid Crystals (Clarendon, Oxford, 1993)] the coupling of the n ematic orientational order parameter to the smectic-A layering order p arameter can substantially alter the critical behavior near the N-SmA transition if the nematic range is small and the nematic order paramet er susceptibility is large. In this paper, we present a direct experim ental comparison of two compounds: 4-octyloxy-4'-cyanobiphenyl (8OCB) with a short nematic range and 4-octyloxybenzoyloxy-4'-cyanotolane (C( 8)tolane) with a very large N range. The temperature variations of the apparent birefringence hn and the specific heat C, across the N-SmA p hase transition show the definite influence of the proximity of the is otropic phase in the case of 8OCB while the C(8)tolane behaves as expe cted for the three-dimensional XY universality class. The elastic modu lus B in the SmA phase, measured at several wave vectors by the second -sound resonance technique, was studied with high resolution as a func tion of temperature on approaching T-c(N-SmA). These elastic data conf irm the B leveling off in both cases with an apparent breakdown of hyd rodynamics in the case of the C(8)tolane compound.