Isothermal melting of near-monolayer xenon on single-crystal graphite

Our study investigates the melting transition of near-monolayer xenon at three temperatures, 121 K, 140 K and 145 K. Xenon on graphite is widely regarded as a model system for the investigation of two-dimensional melting in the presence of a weak orientational field. It has been suggested that it may represent an example in which melting occurs continuously through the unbinding of topological defects in a way analogous to the Kosterlitz-Thouless transition in superfluids. Recent electron diffraction studies are consistent with a two-stage melting process in this system with an intermediate aligned liquid or hexatic phase between the solid and quasi-isotropic liquid phases. We report isothermal high-resolution synchrotron X-ray diffraction measurements of the transition from a finite-size limited 2D solid to a well correlated ( approximately 100 AA) orientationally ordered 2D liquid. Our investigation confirms that at 140 K and 145 K there is a continuous evolution of the length scale of the positional fluctuations up to approximately 1000 AA. At 121 K, despite no discernable evolution of the correlation length, the transition is found to be no sharper in reduced units than at the higher temperatures. The exponent characterizing the decay of the order parameter corresponding to a finite-sized 2D crystal is found to be consistent with values reported for finite-sized two-dimensional magnets undergoing Kosterlitz-Thouless-like (2D-XY) continuous transitions.