1. Steinbuch Centre for Computing, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany
2. Institute of Nanotechnology, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany, and DFG Center for Functional Nanostructures, Department of Physics, Universität Karlsruhe, Wolfgang Gaede Strasse 1, 76131 Karlsruhe, Germany