Long-range adsorbate interactions mediated by a two-dimensional electron gas

We report on long-range interactions between adsorbates on metal surfaces with a surface state. A comparison of three adsorbate/substrate systems Cu/Cu (111), Co/Cu (111), and Co/Ag (111) suggests the general existence of such interactions and shows up common characteristics. In all cases, the interaction energy E (r) manifests itself up to a distance of 60, decays as 1/r^2, and oscillates with a period of ₅/2. Our data are in excellent agreement with theory and establish the link between the spatial variation of the interaction energy and the adsorbate scattering properties. We demonstrate that the long-range interactions stabilize an ordered two-dimensional (2D) gas of adsorbates and thus create states of dilute 2D matter.