1. Department of Physics, Faculty of Electrical Engineering and Communication , Brno University of Technology , Technick 2848/8, 616 00 Brno , Czech Republic
2. Faculty of Physics , Dagestan State University , 367015 Makhachkala , st. M. Gadjieva 43-a, Dagestan Republic, Russia
3. Department of Condensed Matter Physics, Faculty of Science , Masaryk University , Kotlářská 267/2, 611 37 Brno , Czech Republic