Multicriticality in hexatic-liquid crystals

The elegant theories of defect mediated transitions1 provide an interesting scenario for melting of two dimensional crystals. In particular they predict an intermediate phase with quasi-long-range orientational order: the hexatic 2. However, observation of the crystal-hexatic-liquid sequence in either experiments or simulations has proved controversial1 - the hexatic phase can be pre-empted by a direct discontinuous melting transition. Birgeneau and Litster3 observed that certain liquid crystal phases can be regarded as three dimensional stacked hexatic layers. The coupling of layers in the third dimension, endows them with true long-range orientational order, and changes the nature of the corresponding transitions4. However, by looking at successively thinner films of such liquid crystals one may hope to approach the two dimensional limit, and indeed recently several groups have been engaged in such a pursuit, using diffraction studies of freely suspended liquid crystal films5–7. These studies may provide the most accurate and cleanest probe1 of the two dimensional melting sequence.