1. Institute of Applied Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
2. Institute of Biological and Chemical Systems, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
3. Zoological Institute, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
4. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany
5. Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States