Electronic properties of metallic bilayers deposited on Cu(1Â 1Â 1): A comparative study

The nature and the dispersion of the electronic collective excitations in different metal bilayers (Na, Ca, Ag) deposited onto the Cu(1 1 1) surface were investigated by angle-resolved electron energy loss spectroscopy. We found a nearly-flat behavior of the surface plasmon energy (absence of dispersion) in Ca and Ag bilayers, characterized by the presence of d electrons, in good agreement with theoretical predictions within the framework of the s-d polarization model. On the contrary, an initial negative dispersion was observed in the Na bilayer. The intensity of the surface plasmon was vanishing in the long-wavelength limit in all cases.