X-Ray Scattering Study of the Smectic-A1to Smectic-A2Transition

X-ray scattering measurements are reported for critical smectic-$A_2$ fluctuations along a line of second-order transitions between the smectic-$A_1$ and smectic-$A_2$ phases in mixtures of hexylphenyl cyanobenzoyloxy benzoate (D${\mathrm{B}}_6$) and terephthal-bis-butylaniline (TBBA). The measured exponents $\gamma =1.46\mathrm{}\pm 0.05$ and ${\nu }_{\parallel }={\nu }_{\perp }=0.74\mathrm{}\pm 0.03$ are constant along the second-order line and agree with recent heat-capacity measurements and the scaling law, $3\nu +\alpha =2\mathrm{}$. They disagree with current theoretical expectations.