1. a Department of Physics , Queens College of the City, University of New York , Flushing, NY, USA
2. b Donetsk Physico-Technical Institute NASU , Donetsk, Ukraine
3. c Ivan Franko Lviv National University , Lviv, Ukraine
4. d Department of Materials Science and Engineering , Stony Brook, NY, USA
5. e Faculty of Physics , M. V. Lomonosov Moscow State University , Moscow, Russia