Two-stage melting in two dimensions in a system with dipole interactions.

Abstract

Two-stage melting is observed in a two-dimensional colloidal suspension with dipolar interactions. Micrometer-sized polystyrene spheres dispersed in water are confined to two dimensions between glass cover slides. Application of an ac electric field normal to the layer leads to an interparticle interaction that is to good approximation a repulsive dipole interaction. The array solidifies at large applied fields. Translational and bond-orientational correlation functions and critical exponents for the algebraic decay of these correlation functions are consistent with the dislocation-mediated melting theory of Kosterlitz, Thouless, Halperin, Nelson, and Young. The defects observed are those predicted for the crystal, hexatic, and fluid phases. The one-component plasma parameter, \ensuremath{\Gamma}, is found to equal 61\ifmmode\pm\else\textpm\fi{}3 at melting in agreement with computer simulations. Grain boundaries are not observed in the intermediate, hexatic phase. The bond-orientational correlation function shows a clear hexatic-to-fluid transition. The potential energy varies smoothly through the transitions between phases.