Influence of the substrate electronic structure on metallic quantum well state dispersions in ultrathin metal films

Abstract

We have studied ultrathin Cu films grown on three related ferromagnetic (FM) layers to clarify the role of the substrate in determining the two-dimensional dispersion of metallic quantum well (MQW) states in the overlayer. The dispersions with parallel momentum of Cu MQW states above the Fermi level EF were measured in the Cu/fccFe/Cu(1 0 0) and Cu/fccCo/Cu(1 0 0) systems using inverse photoemission, and below EF in the Cu/fccNi/Cu(1 0 0) system using angle resolved photoemission spectroscopy. This study focussed on the ΓX̄ direction of the two-dimensional Brillouin zone. For states away from the projected band gaps of the FM layer, the identity of the FM layer (i.e., Fe or Co) has little influence on Cu MQW states and their dispersions closely resemble those of a free standing Cu film. In contrast, all three systems exhibit nondispersing MQW states when the Cu bands overlap the minority spin projected band gaps of the FM metal. A phase accumulation approach gives a very simple explanation of this behavior, showing that the flat dispersion occurs because the phase shift upon reflection from the FM layer has a strong energy dependence in the projected gap. © 2001 Elsevier Science Ltd. All rights reserved.