Static and dynamic behavior near a second-order smecticA— nematic phase transition by light scattering

Measurements are presented of the intensity and lifetime of Rayleigh light scattered from thermally excited twist and splay modes of $p$-cyanobenzylidine-$p-n$-octyloxyaniline (CBOOA) in the nematic phase near the smectic $A$—nematic phase transition. The temperature dependence of the twist and splay elastic constants and viscosities are deduced. The phase transition is continuous ($T_c-T^{*}<3\mathrm{}°$ mK); this contradicts the prediction of Halperin, Lubensky, and Ma that this phase transition would be weakly first order. The critical behavior of the twist elastic constant $K_{22}$ and the twist viscosity ${\gamma }_1$ is consistent with mean-field predictions from 3°mK to 7°K above the transition; these data are inconsistent with the critical behavior predicted by the helium analogy. The splay mode is not renormalized, as expected.