•Romeike Christian^{1}, Wegewijs Maarten^{1}, Wenzel
Wolfgang^{2} und Schoeller Herbert^{1}
^{1}Institut für Theoretische Physik - Lehrstuhl A, RWTH
Aachen, 52056 Aachen, Germany
^{2}Forschungszentrum Karlsruhe, Institut für Nanotechnologie,
76021 Karlsruhe, Germany

We theoretically describe electron tunneling transport through a transition-metal complex coupled to metallic electrodes. Recently such a system containing a single cobalt ion has been investigated experimentally [1]. Upon increasing the coupling of the electrodes to the complex in a chemically controlled way, a strong Kondo effect in the current was observed at low temperatures.

In order to describe such an experiment, one has to formulate a interacting
electronic model for the transport. We first perform electronic structure
calculations on the isolated cobalt-complex. The symmetry restrictions
on the tunneling from particular types of ligands onto the metal ion are
carefully analysed. We next address the question of which states on the
ion and ligands are to be included in a minimal model for the transport.
Next, the coupling to the leads is taken into account in a phenomenological
way by assuming a tunneling Hamiltonian with tunneling matrix elements
which enter as parameters of our model through electron-tunneling rates.
We address the possible transport situations which may be realized in such
systems when different transition-metals are substituted for the cobalt.