Low-Energy Collective Electronic Mode at a Noble Metal Interface

A novel low-energy collective mode of electrons near the Fermi energy has been detected at 5 meV for (9x9)-Ag/Cu(111) by high-resolution electron energy loss spectroscopy. This collective electronic excitation does not exist on the bare Cu substrate or on a silver monolayer deposited on Ni(111). It has been assigned to the excitation of the Friedel oscillations in the silver adlayer. Such finding is important for understanding dynamic electron screening at a metal/metal interface. In particular, this work provides the grounds for more accurate theoretical studies aimed at describing the surface loss function and low-energy collective electronic modes in Ag/Cu(111).