Two Topics Related to the Conduction Properties of Atomic Wires

We discuss here two topics related to the study of the conductance of short atomic wires: conduction through single Mg and Na atoms linking two electrodes, and negative differential resistance at atomic contacts. For the first topic, we consider the current flow between two planar metallic electrodes with one atom in the region between them. When the electrodes are close enough so that they are both in contact with the atom, they can be though of as being connected by one-atom wire. We discuss the conductance G due to the presence of the atom (i.e. the change of total conductance when the atom is added) as a function of the electrode spacing, with the atom always kept midway between the two electrodes. An experimental approximation to this could be obtained by adsorbing on both tip and sample of a scanning tunneling microscope a layer of spacer atoms or molecules (relatively inert atoms or molecules with essentially no states near the Fermi level), adsorbing the atom of interest on top of one of these layers and then bringing the other electrode up until its spacer layer is also in contact with the atom. We consider the cases of either a single Na or a single Ma atom between the electrodes, and discuss the difference between them. All calculations were done fully self-consistently using the density-functional treatment as described in [1]. The electrodes are represented using the uniform-background (jellium) model, with rs=2 bohrs, typical of a high-electron-density metal such as A1.[2] The cores of the atoms were represented using a pseudopotential.[3]...