DYNAMICS OF THE LOWEST-ORDER BEND-TWIST DIRECTOR MODE NEAR NEMATIC SMECTIC-A CRITICALITY

A heterodyne light-scattering (LS) experiment that directly probes the rmal fluctuations of the fundamental (lowest-order) bend director mode in homeotropic geometry, and a high-resolution Freedericksz-transitio n (FT) experiment which excites the corresponding static mode magnetic ally were performed just above nematic-smetic-A criticality on the non polar wide-nematic-range material dihexylazoxybenzene over the reduced temperature ranges 3 X 10(-7) less-than-or-equal-to t less-than-or-eq ual-to 2.0 X 10(-3) and 2.7 X 10(-5) less-than-or-equal-to t less-than -or-equal-to 2.0 X 10(-2), respectively. It is found that over the nea rly five decades of reduced temperature covered by these experiments, the enhancement of the bend elastic constant, K33, obeys a simple powe r law with a critical exponent rho3 = 0.825+/-0.008 (FT), rho3 = 0.815 +/-0.03 (LS); the error estimates include variations of rho3 upon exte nsive range shrinking and pp expansion of the data sets. These studies cover a wider range of reduced temperature and extend two decades or more closer to the critical point than previous studies. Over the near ly five decades of reduced temperature covered by these experiments th e bend constant K33 increases by nearly four orders of magnitude to 5 X 10(-3) dyn corresponding to a longitudinal correlation length of xi( parallel-to) = 7.9t-0.82 angstrom (congruent-to 200 mum at t = 3 X 10( -7)). The measurement of correlation lengths of this magnitude may be unprecedented in the study of critical phenomena. In spite of accessin g the regime where xi(parallel-to) is comparable with the thickness of our films, approximately 200 mum, finite-size and surface effects are not observed. We also report the results of the above heterodyne expe riment performed on another nonpolar material, hexyloxynonylbenzoate ( 609BAR), over a similar reduced-temperature range. This experiment yie lded the same critical exponent within experimental uncertainty, rho3 = 0.84+/-0.03, contrary to previously reported Freedericksz-transition measurements which exhibited a crossover from approximately 0.84 to 0 .67 on range shrinking and were well represented over the entire exper imental range of reduced temperature by a power-law term with rho3 = 0 .67+/-0.02 augmented by an anomalously large negative correction to sc aling term. This discrepancy is discussed briefly in the context of su rface phenomena and is currently under study. We compare our results w ith theoretical predictions about nematic-smectic-A criticality and wi th the director normal-mode spectrum which we derive from linearized E rickson-Leslie-Parodi theory including the effects of finite-director anchoring and surface dissipation.