1. Institute for Applied Physics and Center for Nanotechnology, University of Muenster, Corrensstrasse 2-4, 48149 Muenster, Germany
2. Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA
3. National Research Nuclear University MEPhI, 115409 Moscow, Russia
4. Institute of Metal Physics, Ural Division of RAS, Yekaterinburg 620041, Russia
5. Department of Physics, Emory University, Atlanta, Georgia 30322, USA