Pressure-Driven Commensurate-Incommensurate Transition in Low-Temperature Submonolayer Krypton on Graphite

By using ${\mathrm{D}}_2$ gas as a source of two-dimensional spreading pressure, we have studied the commensurate-incommensurate (C-I) transition in submonolayer Kr on $\mathrm{ZYX}$ graphite at temperatures near 40 K. High-resolution synchrotron x-ray diffraction results show both hysteresis and C-I phase coexistence, clear signatures of a first-order transition. This agrees with theoretical predictions, but apparently gives different behavior from earlier studies at higher temperatures and coverages.