Mobility of conduction electrons in ultrathin Fe and Cu films on Si(111)

Using in situ transmission spectroscopy in the middle infrared, an enhancement of the Drude-type plasma frequency with respect to the bulk value was found for ultrathin Fe films in < 111 > orientation grown on Si(111)7x7. That finding is in good agreement with ab initio calculations that, in particular, take into account the existence of surface states and surface resonances. These calculations also predict an increase of the plasma frequency with decreasing thickness for Cu(111) films, but the experimental proof for Cu was hampered by an islandlike growth. However, in experiment and in theory the bulk value for the plasma frequency of copper is already reached at 3 nm thickness.