Critical vacancy density for melting in two-dimensions: The case of high density Bi on Cu(111)

The two-dimensional melting/solidification transition of the high density [2012] phase of Bi on Cu(111) has been studied by means of low energy electron microscopy (LEEM). This well defined phase has an ideal concentration of one Bi atom per two Cu surface atoms (θ _Bi = 0.500). The Bi density is determined accurately in situ and the highest melting temperature of 538 K occurs at exactly θ _Bi = 0.500. A significantly reduced melting temperature is observed for lower Bi densities (θ _Bi < 0.500) and, surprisingly, also for θ _Bi > 0.500. At |Δ θ _Bi| = 0.015 the melting temperature is reduced by about 20 K. This lowering of the melting temperature is attributed to a critical vacancy density at melting and we propose that this quantity triggers the 2D solid-liquid phase transition. For this particular system, the critical vacancy fraction for melting amounts to 5%-6%. Above θ _Bi = 0.500 and near melting a homogeneous, unilaterally compressed phase, '[2012]' is observed, with a density that increases continuously with coverage. It is commensurate along and incommensurate along The ability to distinguish between Bi accommodated within the '[2012]' phase and Bi residing on top as a lattice gas by applying LEEM is of crucial importance for the analysis.