1. Institute of Physics and Center for Nanotechnology, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
2. Institute of Solid State Theory, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
3. Institute of Mineralogy, University of Münster, Corrensstr. 24, 48149 Münster, Germany