Commensurate melting, domain walls, and dislocations

Commensurate phases of order p 3 exhibit two or more classes of inequivalent domain walls, reflecting a lower than ideal symmetry. These walls compete statistically and undergo wetting transitions. New "chiral" universality classes of melting transitions may thereby occur for both 3×1 and 3×3 surface phases. The data of Moncton et al. may be interpreted as indicating that such a chiral transition occurs in Kr on graphite. The melting of p×1 phases is discussed for various dimensionalities d and values of p. Domain-wall wetting transitions are treated in a semiphenomenological fashion; they may be either continuous or first order. Wetting critical exponents are obtained for a general class of transitions. The role of dislocations at the uniaxial commensurate-to-incommensurate transition is examined. For d=2 the crossover exponent for dislocations is found to be - p=(6-p2)4. For p>6 the dislocations are therefore irrelevant, but they introduce singular corrections to scaling at the transition. A phase diagram as a function of dislocation fugacity is proposed for the case d=2, p=3, illustrating how a Lifshitz point may be present at all nonzero fugacities.