Rayleigh Scattering at a Second-Order Nematic to Smectic-APhase Transition

We observe a second-order nematic to smectic-$A$ phase transition in $p$-nitrilebenzylidene-$p$-octyloxyaniline using the Rayleigh scattering technique. The twist elastic constant $K_{22}$ diverges at the transition, with the critical exponent $\nu =0.66\mathrm{}\pm 0.05$ in agreement with de Gennes's prediction. For large-angle scattering we observe the onset of the critical regime.