1. Walter Schottky Institut and Physik Department, Technische Universität München, 85748 Garching, Germany
2. Nanosystems Initiative Munich (NIM), 80799 Munich, Germany
3. Department of Physics, Columbia University, New York, New York 10027, USA
4. Institut für Theoretische Physik, Universität Bremen, 28334 Bremen, Germany
5. Bremen Center for Computational Materials Science, University of Bremen, 28359 Bremen, Germany
6. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
7. National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
8. Munich Center for Quantum Science and Technology (MCQST), 80799 Munich, Germany
9. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
10. Institute of Physics, University of Münster, 48149 Münster, Germany