1. Computer Solid State Physics Laboratory, University of Aizu, Aizu- Wakamatsu 965-8580, Japan
2. Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
3. Department of Electrical Engineering, University at Buffalo, SUNY, Buffalo, NY 14260, USA
4. Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA