Dislocation Mediated Melting Near Isostructural Critical Points

We study the interplay between an isostructural critical point and dislocation mediated two-dimensional melting, using a combination of Landau and continuum elasticity theory. If dislocations are excluded, coupling to the elastic degrees of freedom leads to mean field critical exponents. When dislocations are allowed, modified elastic constants lead to a new phase buried in the solid state near the critical point. Consistent with a proposal of Bladon and Frenkel, we find an intervening hexatic phase near the critical point of the first-order isostructural transition line. Very close to the critical point, a transition from the hexatic phase to a modulated bond angle state is possible. Ising transitions in symmetrically confined two dimensional colloidal crystals with attractive wall potentials require a different model free energy, which we also discuss.