1. Ivan Franko National University of Lviv, Lviv 79017, Ukraine
2. Institute of Solid State Physics, University of Latvia, Riga LV-1063, Latvia
3. Jan Dlugosz University in Czestochowa, Czestochowa 42-201, Poland
4. Department of Specialized Computer Systems, Lviv National Polytechnic University, Lviv 79013, Ukraine
5. Kryvyi Rih State Pedagogical University, Kryvyi Rih 50086, Ukraine
6. Institute of Physics, University of Tartu, Tartu 50411, Estonia