Two-stage melting is observed in a confined monolayer of colloidal particles with dipole interactions.The defects observed are consistent with those predicted for the crystal, hexatic, and fluid phases. The bond-orientational correlation function shows a clear hexatic-to-fluid transition, and the ensemble's potential energy shows no obvious discontinuity around either transition. PACS numbers: 64.70.Dv, 61.72.Lk, 64.60.Cn, 82.70.Dd Past investigators of melting in two dimensions have been equivocal about whether this phase transition proceeds by a continuous two-stage process as developed by Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY) [1 -3] or by a first-order process in which the solid transforms directly into a liquid, coexisting with this state at the transition.Compelling evidence for the KTHNY theory in systems with long-range interparticle potentials has been presented from work on electrons on the surface of helium [4], and submicron polymer colloids confined between glass plates [5]. A single KosterlitzThouless type disclination unbinding has been seen in a simulated thermal system using magnetic bubbles in a thin garnet film [6). Evidence of a first-order transition, however, has been noted in systems with short-range interparticle potentials, specifically in careful Monte Carlo simulations of hard spheres in two dimensions [7] and from specific heart measurements of xenon on grafoil [8].Outright equivocal results have been obtained from micron-sized colloidal monolayers [9] for which the interparticle should be of shorter range than that of the submicron colloids. All this suggests that melting in two dimensions is not governed by a simple universality but is dependent on the form of the interparticle potential, a position also implied by a Monte Carlo simulation of a vector Coulomb gas [10].Our experiment [11] explores the melting mechanism of the dipole potential in a substrate-free, two-dimensional system.It provides evidence of a two-stage melting process in which the translational and orientational correlation functions and the defect structures are consistent with KTHNY. The system studied is composed of a monolayer of 1.6 p, m, polystyrene colloidal spheres in water [12] confined between two glass plates separated approximately by one and one-half particle diameters. A high frequency electric field applied normal to the monolayer polarizes the polymer/water interface, creating, to first-order approximation, a dipole interparticle potential that scales as the square of the applied field. Since the dipoles are aligned with the external field and confined to a plane, the particle interaction is a scalar r potential. This system has the advantages of being composed of particles large enough to allow direct imaging of the colloidal monolayer with optical microscopy, yet small enough to create a quasithermodynamic system of )104 particles which equilibrates on the order of hours. Also, the experiment probes a potential that is intermediate to the long-range Coulomb potential and sh...
