1. Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
2. Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
3. Institute of Applied Physics (APH), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
4. Department of Applied Physics, Aalto University, PO Box 13500, FI-00076 Aalto, Finland