IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Wednesday Sessions
       Session SS3-WeA

Paper SS3-WeA6
Quantum Well Behavior without Confining Barrier Observed via the Dynamically Screened Photon Field

Wednesday, October 31, 2001, 3:40 pm, Room 122

Session: Electronic Structure I
Presenter: P. Haberle, Universidad Tecnica Federico Santa Maria
Authors: S.R. Barman, Inter-University Consortium for DAE Facilities
P. Haberle, Universidad Tecnica Federico Santa Maria
K. Horn, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
J. Maytorena, Universidad Nacional Autonoma de Mexico
A. Liebsch, Forschungszentrum Julich, Germany
Correspondent: Click to Email
Quantum well states are a striking manifestation of elementary quantum mechanics. An electron confined in a one-dimensional potential well, formed by the vacuum on one side and a band gap in the substrate on the other, may occupy discrete energy levels whose quantum number specifies the number of half-wavelengths spanning the well. Angle resolved photoemission spectra from Na adlayers on Al(111) reveal features which behave as quantum well resonances, even though the substrate does not provide a barrier for electron confinement. These features are observed in a narrow photon energy range where overlayer collective excitations cause resonant enhancement of the photoemission intensity. The quantum well behavior is shown to be due to surface resonances of the Na/Al system. These are observable since the screened photon field is dynamically enhanced and spatially confined to the overlayer. The special role of the photon field distinguishes the present system from ordinary quantum wells. The quantum well-like spectral features are associated with virtual states induced by the large negative potential step between overlayer and substrate. To observe these resonances it is crucial to suppress emission from substrate bands in the same energy region. This is achieved by tuning the photon energy to the collective modes of the overlayer. The local electric field is then enhanced and confined to the overlayer so that the Na resonances can be observed in a wide range of coverages. Thus, while in usual quantum well systems the photon field plays no special role, here it serves as a novel mechanism ensuring the confinement of the excitation region.