1. Department of Electrical Engineering and Bioscience Waseda University 3‐4‐1 Ohkubo, Shinjuku‐ku, Tokyo 169‐8555 Japan
2. Graduate School of Information, Production and Systems Waseda University 2‐7 Hibikino, Wakamatsu‐ku, Kitakyushu, Fukuoka 808‐0135 Japan
3. Research Institute for Materials Science and Technology Waseda University 3‐4‐1 Ohkubo, Shinjuku‐ku, Tokyo 169‐8555 Japan
4. Advanced Technology Research and Development Center Mitsubishi Electric Corporation 8‐1‐1 Tsukaguchi, Honmachi, Amagasaki Hyogo 661‐8661 Japan