Halperin and Nelson propose that the melting of a 2D solid involves an intermediate phase with both liquidlike and solidlike properties. The discovery of this so-called hexatic phase, which is later observed in experiments and simulations, demonstrates that melting in 2D systems (like thin films) is fundamentally different from that in 3D materials.
About hexatic phase:
The hexatic phase is a phase that is between the solid and the isotropic liquid phases in two dimensional systems of particles. It is characterized by two order parameters: a short-range positional and a quasi-long-range orientational (sixfold) order. More generally, a hexatic is any phase that contains sixfold orientational order, in analogy with the nematic phase (with twofold orientational order).
