Theory of Scanning Tunneling Spectroscopy of Kondo Ions on Metal Surfaces

Due to the Kondo effect, a single magnetic ion in a metallic host can produce a narrow resonance at the Fermi level in the spectral density of the ion’s d- (or f-) orbital as well as in the local conduction electron density of states (c-LDOS) [1]. However, only recently sufficient spatial and energy resolution could be achieved by scanning tunneling microscope (STM) spectroscopy to directly measure the LDOS correction induced by a single magnetic ion adsorbed on a metal surface [2, 3, 4] (see Fig. 1, inset). The electronic density of states, as measured by the dI/dV characteristics of the STM, shows a narrow resonance at the Fermi level whose asymmetric line shape resembles that of a Fano resonance [5] and changes with the distance R from the ad-atom.