1. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
2. Center for Functional Nanostructures, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
3. Fraunhofer Institute for Applied Solid State Physics, Tullastr. 72, 79108 Freiburg, Germany