1. Amirkhanov Institute of Physics, Daghestan Scientific Center, Russian Academy of Sciences (RAS), 367003 Makhachkala, Russia
2. Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala SE-75120, Sweden
3. Department of Physics, Belgorod State University, RUS-308015 Belgorod, Russia
4. South-West State University, Regional Centre of Nanotechnology, 50 Let Octjabrja Str. 94, 305040 Kursk, Russia