1. Institute of Bioelectronics (PGI-8/ICS-8) and JARA—Fundamentals of Future Information Technology, Forschungszentrum Jülich, 52425 Jülich, Germany
2. MESA+ Institute for Nanotechnology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands
3. Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany