First-principles investigation of the quantum-well system Na on Cu(111)

The alkali-metal adsorption system Na on Cu(111) has been investigated theoretically in order to clarify the possible presence of quantum-well states observed in photoemission experiments. We analyze the site preference, bonding character, the coverage dependence of the work function, and the electron structure of the system. The study is based on first-principles calculations of free-standing Na layers in vacuum, the clean Cu(111) surface, and the Na/Cu(111) adsorption system at two different adsorbate structures: (2 x 2) and (3/2 x 3/2). We are able to identify Na-induced bands, which are essentially localized in the local band gap of the Cu(111) surface. The local density of stares at the <(Gamma)over bar> point for the lowest of these bands has maxima within the Na adlayer and decays rapidly into the substrate. The dispersion of the Na-induced bands is in good agreement with photoemission data and the <(Gamma)over bar> point energy of the lowest Na-induced band shifts from 0.45 eV above the Fermi level for the (2 x 2) structure to 0.06 eV below the Fermi level for the saturated monolayer. We conclude that the Na-induced states have the characteristics of quantum-well states, which supports previous interpretations based on photoemission experiments.