Light-Scattering Study of a Smectic-APhase near the Smectic-A- Nematic Transition

From light scattered by director modes in the smectic-$A$ phase of cyanobenzylidene octyloxyaniline (CBOOA) and octyloxy cyanobiphenyl (80CB) we deduce that the restoring force for phase fluctuations in the smectic order parameter vanishes as ${(T_c-T)}^{0.33}$ as the nematic phase is approached. In CBOOA the force maintaining the molecules normal to the layers vanishes as ${(T_c-T)}^{0.50}$. We conclude that de Gennes's analogy between charged superfluids and the smectic-$A$ phase is not entirely correct.