Free Electron Theory for Thin Metal Films

Quantum free electrons, i.e. plane waves, with wavevector k, and occupancy constrained by the Pauli exclusion principle, are explained in all introductory texts about solids. A free-electron description works surprisingly well for many properties of "simple" metals. It is assumed that the interior of the metal is essentially infinite, and surfaces are presumed irrelevant. Over the past 30 years, experiments that visualize surfaces have revolutionized solid state physics, stimulating new theory and applications. Therefore, a basic question is, how can the free electron picture be applied to properties of solids where surfaces play a prominent role? Various versions of an extended free-electron theory are used, but not always explained pedagogically. This paper focusses on idealized metallic films. Three versions (an oversimplified one and two stages of improvement) of a free-electron description of metal films are given. These versions are illustrated in detail for the specific example of a slab of aluminum with six layers of atoms.