Experimental evidence for the breakdown of conventional elasticity in smectics A

The birefringence ⌬n, the specific heat C p , and the layer compressional elastic modulus B are reported for two liquid crystals near the nematic (N) to smectic-A (SmA) 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, 1993͔͒ the coupling of the nematic orientational order parameter to the smectic-A layering order parameter can substantially alter the critical behavior near the N-SmA 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-4Ј-cyanobiphenyl ͑8OCB͒ with a short nematic range and 4-octyloxybenzoyloxy-4Ј-cyanotolane (C 8 tolane) with a very large N range. The temperature variations of the apparent birefringence ⌬n and the specific heat C p across the N-SmA phase transition show the definite influence of the proximity of the isotropic phase in the case of 8OCB while the C 8 tolane behaves as expected for the three-dimensional XY universality class. The elastic modulus B in the SmA phase, measured at several wave vectors by the secondsound resonance technique, was studied with high resolution as a function of temperature on approaching T c (N-SmA). These elastic data confirm the B leveling off in both cases with an apparent breakdown of hydrodynamics in the case of the C 8 tolane compound.

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