1. Institute of Physics, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
2. Center for NanoTechnology (CeNTech), Heisenbergstr. 11, D-48149 Münster, Germany
3. Center for Soft Nanoscience (SoN), Busso-Peus-Str. 10, D-48149 Münster, Germany
4. B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 61103 Kharkiv, Ukraine