Anomalous thickness dispersion of unoccupied states in the Cu/Ni/Cu(100) metallic quantum well system

We present photoemission and inverse photoemission measurements of the occupied and unoccupied electronic states of the Cu/Ni/Cu(100) system. While the occupied levels of the Cu overlayer display the usual upward dispersion with increasing overlayer thickness, the unoccupied states exhibit an anomalous downward dispersion similar to that seen for the Cu/Ni(100) system. While the anomalous dispersion depends quantitatively on Ni film thickness, and hence on in-plane strain, the qualitative behavior of downward dispersing features is seen for all Ni thicknesses studied. Our results suggest that the origin of the anomalous dispersion is tied to the Cu-overlayer/Ni-film interface.