Electron localization in metallic quantum wells: Pb versus In on Si(111)

Two-dimensional quantum well states in ultrathin metal films generally exhibit a dispersion relation of s-p-derived states that can be described through an effective mass of the corresponding bulk band. By contrast, the effective masses in Pb quantum well states on Si(111), measured through angle-resolved photoemission, are up to an order of magnitude larger than those from the bulk states or predicted by slab calculations, while similar anomalies are not observed in the related In/Si(111) system. We interpret these data in terms of an enhanced electron localization, and use them to interpret recent scanning tunneling microscopy results.