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

Abstract

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 á ñ -11 2 and incommensurate along á ñ -1 10 . 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.

Collective phenomena on metallic surfaces studied with scanning tunneling microscopy and low energy electron microscopy

“…This suspicion has been confirmed with the current result. We wish to note the resemblance of the model in Figure 7.9(B) to the 2 0 1 2 phase observed for Bi deposited onto Cu(111) [7,37,[216][217][218]. For deposition of Bi on Cu (111), for low coverages and low temperatures, Bi is incorporated in the first layer in substitutional √ 3 alloy phase with compressive stress.…”

“…Therefore, we alternatively looked at the Bi-modified Ni (111) surface. Bi deposition on the Ni (111) and Cu (111) surfaces has already shown to lead to a variety of dynamic processes [2][3][4][5][6][7]. Therefore, the Bi/Ni system was a perfect playground for our study, with the added value that we can further expand our knowledge of its surface phase diagram.…”

“…For the Bi/Ni(111) system, again, Bollmann et al [2][3][4] have formerly demonstrated exotic phenomena such as nanowire self-assembly and quantum growth of film structures. Also, for the energetically similar Cu (111) surface, Bi deposition to different coverages leads to a quite interesting phase diagram, including 2D melting, a liquid phase, a droplet phase incorporated into a solid matrix, resonant scattering of low energy electrons, and more [5,7,37]. Inspired by the versatility in these systems and the capabilities of the low energy electron microscope, we have explored the Bi/Ni (111) surface with a new set of experimental parameters, compared to the former studies of Bollmann et al [2][3][4].…”

“…To observe the sample, we sometimes shine light through the vacuum system look-through windows, the lamp (5) has extended LED sources for easier illumination of the desired area. A valve (6) separates the preparation chamber from the main STM chamber (7), where the measurements are performed. The sample is positioned on a stage, controlled with a feed-through knob (8).…”

“…The function of the magnetic beam splitter/separator (5) is to separate the incoming electron beam from the backscattered beam. After the separator, the incoming beam is sent to the main chamber of the LEEM (6), where the sample is mounted (7). A cathode objective lens (OL, 8) directs the electrons to the sample surface.…”