1. Faculty of Physics and Technology, L.N. Gumilyov Eurasian National University, Astana 010000, Kazakhstan
2. Faculty of Radiophysics and Computer Technologies, Belarusian State University, Kurchatova Street 5, 220045 Minsk, Belarus
3. Institute of Solid State Physics, University of Latvia, 8 Kengaraga, LV-1063 Riga, Latvia
4. Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia