1. Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany
2. Institute of Biophysics, University of Ulm, Albert Einstein-Allee 11, 89081 Ulm, Germany
3. Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States