1. Max Planck Institute for Intelligent Systems, 70569 Stuttgart, Germany
2. Department of Physics, Universität Konstanz, 78464 Konstanz, Germany
3. Okinawa Institute of Science Technology Graduate University, 904-0495 Okinawa, Japan
4. Institute of Solid State Physics, Ulm University, 89081 Ulm, Germany
5. Institute of Physics, Johannes Gutenberg University, 55122 Mainz, Germany
6. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
7. Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, 47057 Duisburg, Germany