Observation of the Smectic-$C$—Smectic-$I$ Critical Point

We report the first observation of the smectic-$C$—smectic-$I$ ($C-I$) critical point by x-ray diffraction studies on a binary system. These results are in agreement with the theoretical idea of Nelson and Halperin that coupling to the molecular tilt should induce hexatic order even in the $C$ phase, and as such both $C$ and $I$ (a tilted hexatic phase) should have the same symmetry. The results provide evidence in support of the recent theory of Defontaines and Prost proposing a new universality class for critical points in layered systems.