1. Research Institute for Electrical Communication, Tohoku University, 980-8577 Sendai, Japan
2. Computational Nanoelectronics Laboratory, University of Aizu, 965-8580 Aizu-Wakamatsu, Japan
3. Department of General Physics, Moscow Institute of Physics and Technology, 141-700 Dolgoprudny, Russia
4. Department of Electrical Engineering, University at Buffalo, Buffalo, 1460-1920 NY, USA