The melting transition of Xe monolayers adsorbed on a single-crystal exfoliated graphite substrate has been studied by high-resolution synchrotron x-ray scattering. At temperatures slightly above the melting transition the fluid phase has a high degree of orientational order. The results are discussed in the context of current theories of two-dimensional melting including the effects of the substrate. finite density of unbound defects causing an exponential decay of order with the

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... length being the average separation between unbound defects. Defect-mediated transitions can be crudely divided into two generic classes. Transitions in the 2D XY model, superfluid, superconductor, Coulomb gas, and roughening (solid-on-solid) model belong to one class. ' This class of problems is characterized by a scalar interaction between defects. In general, these problems contain no cubic in-32 7373