1. Institute of Pharmacy and Molecular Biotechnology, Heidelberg University, Im Neuenheimer Feld 364, Heidelberg 69120, Germany, Institute of Applied Physics and Center for Functional Nanostructures, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany, and Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801