1. Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
2. Institut für Nanotechnologie, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany
3. University of Potsdam, Institute of Physics and Astronomy, 14476 Potsdam-Golm, Germany