Specific-heat measurements near the nematic-smectic-$A$ transitions of the octyloxy, nonyloxy, and decyloxy members of the 4-$n$-pentyl-phenylthiol-4′-alkoxybenzoate ($\overline{n}\mathrm{S}5$) homologous series reveal the presence of critical-tricritical crossover behavior upon increasing $\overline{n}$, with ${\overline{n}}_t\sim \overline{9}$. The results are consistent with Alben's extension of de Gennes's superfluid analogy to include superfluid-${\mathrm{He}}^3$-${\mathrm{He}}^4$-like tricritical behavior.

• Received 13 August 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.1171