1. Institut für Physik, Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany
2. Department of Physics, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland
3. Research Institute of Physics, Southern Federal University, Stachki Ave. 194, 344090 Rostov-on-Don, Russia