1. Faculty of Physics, Center for
Nanointegration Duisburg-Essen, University of Duisburg-Essen, 47048 Duisburg, Germany
2. Departments of Physics and Electrical
Engineering, Columbia University, New York,
New York 10027, United States
3. Institut für Festkörperforschung and Institute for Advanced Simulation, Forschungszentrum Jülich, 52425 Jülich, Germany