1. Physik Department E20, Technische Universität München, James-Franck-Straße,
85748 Garching, Germany
2. Department of Physics, Chemistry
and Biology (IFM), Linköping University, 58183 Linköping, Sweden
3. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen,
Germany
4. IPCMS-CNRS, Université de Strasbourg, 23 Rue de Loess, 67034 Strasbourg, France