1. Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia
2. Novosibirsk State University, 630090 Novosibirsk, Russia
3. Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
4. Experimentelle Physik 2, Technische Universität Dortmund, 44227 Dortmund, Germany