Hexatic

Hexatic is a phase of matter observed in two-dimensional systems. It is characterized by quasi-long-range bond-orientational order with sixfold symmetry and short-range translational order. The hexatic phase was predicted by the Kosterlitz-Thouless-Halperin-Nelson-Young theory as an intermediate state in the melting of two-dimensional crystals, lying between a solid, which retains quasi-long-range translational order and orientational order, and a liquid, which lacks long-range order in both.

In hexatic, bond-orientational order parameter psi6 shows quasi-long-range order; translational order is short-range. The bond-orientational correlation function G6(r) decays algebraically with distance r, rather than exponentially. The phase is stabilized by bounded topological defects: dislocations drive the solid-to-hexatic transition, while disclinations drive the hexatic-to-liquid transition.

Experimental realizations include monolayers of colloidal particles at fluid interfaces, dusty plasmas, and vortex matter in superconductors; hexatic order has also been reported in lipid membranes and granular systems. Experiments use image analysis of particle positions to measure psi6 and G6(r). The concept provides a framework for understanding two-dimensional melting and the role of defects in ordering.