1. Institute of Solid State Physics, Chernogolovka, Russia a Kazan branch of The Moskow power-engineering institute, Kazan, Russia
2. Institute of Solid State Physics, Chernogolovka, Russia
3. Kazan branch of The Moskow power-engineering institute, Kazan, Russia
4. Physical-Technical Institute of the Russian Academy of Sciences, Sibirsky tract 10/74, 20029 Kazan, Russia