1. Institute of Experimental Physics, Universität Hamburg, Luruper Chaussee 149, Hamburg D-22761, Germany
2. Deutsches Elektronen-Synchrotron, Notkestrasse 85, Hamburg D-22607, Germany
3. Institute for Nanostructure and Solid State Physics & Center for Hybrid Nanostructures, Universität Hamburg, Luruper Chaussee 149, Hamburg D-22761, Germany