1. Institut für Nanotechnologie, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
2. Physikalisches Institut, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany
3. Present address: Institut Néel, CNRS-UJF Grenoble, BP 166, 38042 Grenoble, France
4. School of Computer Science, The University of Manchester, Manchester M13 9PL, U.K.
5. Department of Materials Science and Engineering and Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3108, United States
6. Institut für Mikrostrukturtechnik, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
7. Institut für Festkörperphysik, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
8. DFG Center for Functional Nanostructures (CFN), 76028 Karlsruhe, Germany
9. Institut für Physikalische Chemie, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany