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 Xray diffraction studies on a binary system. This is in confirmity 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. Smectic C (C) and Smectic I (I) are liquid crystalline phases in which the molecules are tilted with respect to the layer normal. In the I phase the tilt order is supplemented by a six-fold bond orientational (BO) order. Nelson and Halperin[1] predicted that coupling to molecular tilt should induce long-range BO order even in the C phase. This has been confirmed by synchrotron Xray studies[2]. Since only the magnitude of BO order is different, both C and I phases have the same 1