1. Institute of Physics and Center for Nanotechnology, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
2. Center for Soft Nanoscience, University of Münster, Heisenbergstr. 11, 48149 Münster, Germany
3. Kirchhoff-Institute for Physics, University of Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
4. Department of Physics, TU Dortmund University, Otto-Hahn-Str. 4, 44227 Dortmund, Germany