1. Department of Solid State Electronics, V.A. Fock Institute of Physics, St. Petersburg State University, 7/9 Universitetskaya nab., 199034 Saint Petersburg, Russia
2. Department of Physics, Omsk State Technical University, 11 Mira prosp., 644050 Omsk, Russia
3. Institute of Physics and Mathematics, Komi Science Centre, Ural Branch of the Russian Academy of Sciences, 167982 Syktyvkar, Russia