1. Institute of Physics and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Heinrich-Plett-Strasse 40, 34132 Kassel, Germany
2. Leibniz Institute of Surface Engineering (IOM), Permoserstrasse 15, 04318 Leipzig, Germany
3. Department of Physics, Free University Berlin, Arnimallee 14, 14195 Berlin, Germany
4. Max Planck Institute for Plasma Physics, Wendelsteinstrasse 1, 17491 Greifswald, Germany