1. Karlsruhe Institute of Technology (KIT), Institute of Nanotechnology (INT); Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany
2. Fraunhofer Institute of Applied Solid State Physics (IAF); Tullastr. 72 79108 Freiburg Germany
3. University of Muenster, Institute of Physics; Wilhelm Klemm-Str. 10 48149 Muenster Germany