Simple model for scanning tunneling spectroscopy of noble metal surfaces with adsorbed Kondo impurities

A simple model is introduced to describe conductance measurements between a scanning tunneling microscope tip and a noble metal surface with adsorbed transition metal atoms which display the Kondo effect. The model assumes a realistic parametrization of the potential created by the surface and a $d_{{3z}^2-r^2}$ orbital for the description of the adsorbate. Fano line shapes associated with the Kondo resonance are found to be sensitive to details of the adsorbate-substrate interaction. For instance, bringing the adsorbate closer to the surface leads to more asymmetric line shapes while their dependence on the tip distance is weak. We find that it is important to use a realistic surface potential to properly include the tunneling matrix elements to the tip and to use substrate states which are orthogonal to the adsorbate and tip states. An application of our model to Co adsorbed on Cu explains the difference in the line shapes observed between Cu(100) and Cu(111) surfaces.