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