1. Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen, Germany
2. Physikalisches Institut, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany
3. Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany