LIGHT-SCATTERING STUDY OF TWIST FLUCTUATIONS NEAR THE NEMATIC SMECTIC-A TRANSITION OF DIHEXYLAZOXYBENZENE

It is well known that the bend and twist elastic constants contain a term that diverges at the nematic-smectic-A phase transition according to a simple power law in reduced temperature; i.e., as & where t=(T-TN-Sm-A)/TN-Sm-A. We report here a high-resolution light-scattering measurement of the twist constant exponent 2 (=0.660.03) in the four-decade range -6.5<logt<-2.5. Within experimental uncertainty, this is the correlation length exponent predicted by the well-known three-dimensional XY model, a result originally predicted by de Gennes [Solid State Commun. 10, 753 (1972); Mol. Cryst. Liq. Cryst. 21, 49 (1973)] and later by dislocation-mediated melting models due to Helfrich [J. Phys. (Paris) 39, 1199 (1978)] and Dasgupta and Halperin [Phys. Rev. Lett. 47, 1566 (1981)]. This is the largest value of 2 reported to date and by comparison with previous results reveals a clear trend of increasing 2 with increasing nematic range. This trend suggests that smaller values of 2 observed for previously studied materials, may in fact be effective exponents associated with critical-tricritical crossover behavior but leaves open the question of whether there exists a universal value of 2. © 1990 The American Physical Society.