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