Electronic properties of metallic bilayers deposited on Cu(1 1 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.