1. Department of Physics, University of Regensburg, Regensburg D-93053, Germany
2. Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
3. Department of Physics, Technical University of Munich, 85748 Garching, Germany
4. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-004, Japan
5. Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-004, Japan