1. Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
2. Institute of Physics II, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
3. Physics of Interfaces and Nanomaterials, MESA+ Research Institute, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands