1. Taras Shevchenko National University of Kyiv, Volodymyrs'ka 64, 01601 Kyiv, Ukraine
2. Institute of Physics of the National Academy of Science of Ukraine, Nauky ave. 46, 03028 Kyiv, Ukraine
3. National Pedagogical Dragomanov University, Pirogova 9, 01030 Kyiv, Ukraine
4. University of Bergen, Allegaten 55, PO Box 7803, 5020 Bergen, Norway
5. Institute for Single Crystals of the National Academy of Science of Ukraine, Lenin ave. 60, ^61001 Kharkiv, Ukraine