We report x-ray scattering measurements around the smectic-$A_1$-to-nematic phase transition of ${\mathrm{C}}_8$${\mathrm{H}}_{17}$O-φ-OOC-φ-${\mathrm{OCH}}_2$-φ-CN, where φ is the phenylene functional group. Critical fluctuations occur at the smectic-$A_1$ monolayer wave vector ${\mathrm{q}}_2$ and noncritical bilayer fluctuations at the incommensurate wave vector ${\mathrm{q}}_1$≊${0.56\mathrm{q}}_2$. This is different from the situation at the surface where the two wave vectors are commensurate with ${\mathrm{q}}_1$${=(1/2\mathrm{q}}_2$. For both types of layering the correlation lengths for the bulk were found to be equal to the penetration depths at the surface. The critical behavior of the monolayer fluctuations is described by the critical exponents γ for the susceptibility and ${\nu }_{?}$ for the correlation length, which are both close to their tricritical values. This is unexpected for the large nematic temperature range involved.

• Received 22 June 1987

DOI:https://doi.org/10.1103/PhysRevA.37.1335