Continuous crystallization in hexagonally ordered materials.

We demonstrate that the phase transition from columnar-hexagonal liquid crystal to hexagonal-crystalline solid falls into an unusual universality class, which in three dimensions allows for both discontinuous transitions as well as continuous transitions, characterized by a single set of exponents. We show by a renormalization group calculation (to first order in =4-d) that the critical exponents of the continuous transition are precisely those of the XY model, giving rise to a continuous evolution of elastic moduli. Although the fixed points of the present model are found to be identical to the XY model, the elastic compliance to deformations in the plane of hexagonal order, mu, is nonetheless shown to critically influence the crystallization transition, with the continuous transition being driven to first order by fluctuations as the in-plane response grows weaker, micro-->0.