The apex atom plays a key role in STM tunneling, in particular if subtle effects like atomic and chemical resolution on metal alloys are considered. In order to provide comprehensive data on the electronic structure of realistic tips we have calculated tungsten (100) films with single apex atoms by first principles molecular dynamics and full potential methods. Molecular dynamics using ultrasoft pseudo potentials has been used to determine the relaxation of the surface layers. The electronic structure of the relaxed film has been calculated with a first principles full potential method, which is most suitable to reproduce the subtle surface effects due to alloying. The calculations suggest that a correct result for the current and corrugation values in the perturbation approach can only be obtained by including the full electronic structure of the tip.
Corresponding author: W.A. Hofer. Reprints also available from P. Varga (varga).