1. Lviv Polytechnic National University, Lviv, Ukraine;
2. Institute of Physics, Polish Academy of Sciences, Warsaw, Poland;
3. Ivan Franko National University of Lviv, Lviv, Ukraine;
4. Institute of Solid State Physics, University of Latvia, Riga, Latvia;
5. Institute of Physics, University of Bydgoszcz, Bydgoszcz, Poland