Electronic States of Metallic Superlattices and Quantum Wells

Quantum wells and superlattices are examples of artificially-produced material structures exhibiting novel electronic properties. The ability to grow such structures with the atomic precision afforded by molecular-beam-epitaxy gives the solid-state experimentalist exciting opportunities to study interesting quantum-electronic effects and to gain new fundamental understanding of material properties. There are many examples of such work involving semiconductor materials, such as GaAs and Al_xGa_1-xAs; however, the basic electronic effects associated with these artificial structures should be found in metallic systems as well. We have examined a variety of such metallic systems with angle-resolved photoemission, a direct probe of electronic band structure. Quantum-well electronic states were observed for Ag(111) overlayers on several different metallic substrates, including Au, Cu, and Ni. In these cases electron confinement is made possible by band structure mismatch between the overlayer and the substrate. The states were observed in Ag films as thick as 40 monolayers (ML), indicating that they have a rather long coherence length. Advantage can be taken of this to apply photoemission to the study of buried interfaces. This is demonstrated through an analysis of data from Ag/Cu systems, in which the reflection phase shift of Bloch electrons at the Cu/Ag interface is determined. For superlattices, systems of alternating slabs of Ag(111) and Au(111) of various configurations with periods ranging from 8 to 12 monolayers were studied. The superlattice bands derived from the sp states of the constituent materials were directly mapped along the (111) direction. They clearly exhibited the fundamental electronic effects of superlattice formation, that is, folding according to the expanded periodicity and the formation of energy gaps at the new zone boundaries. The bands were dispersive even above the Au band maximum, where electronic states would be nonpropagating in bulk Au, indicating that the system cannot be regarded as a collection of noninteracting quantum wells of the type mentioned above.