First-order and continuous melting in a two-dimensional system: Monolayer xenon on graphite

We have performed a high-resolution x-ray scattering experiment to study the two-dimensional melting of xenon adsorbed on exfoliated graphite near the first-order–to–second-order multicritical point for coverages at and below 1 ML (ML denotes monolayer). We have crossed the liquid-solid phase boundary at three points by varying the vapor pressure at fixed temperature. At 116 K we find a weak first-order transition. At 125 and 134 K, which correspond to coverages of about 0.9 and 1.0 ML, respectively, the transition is continuous within experimental accuracy. Fits of the data in the solid phase to theories of dislocation-mediated melting yield η($P_m$)=0.35±0.02 at both 125 and 134 K. From hydrodynamic scaling in the liquid phase, we find η($P_m$)=0.24±0.02 at 116 K and η($P_m$)=0.23±0.02 at both 125 and 134 K. The discrepancy between the two methods for obtaining η($P_m$) cannot be simply explained. Indirect evidence for an orientationally ordered phase is also observed at 125 and 134 K through the azimuthally averaged diffraction line shapes.