Abstract:
Depending on the preparation method, we find two different structures of the Pb/Ge(111) system at a nominal coverage of 4/3 monolayer that exhibit different melting points. One is the well studied beta phase that melts at 270 degreesC, but the other is a new and metastable phase that melts at 330 degreesC. Using surface x-ray diffraction the atomic structure of both phases is found to be surprisingly similar. The difference in melting points can be explained by the distribution of the excess Pb present on the surface, which has a direct effect on the vacancy density. We propose a modified phase diagram, in which the melting temperature of the beta phase depends strongly on coverage.