1. Department of Electrical Engineering and Bioscience Waseda University 3‐4‐1 Ohkubo, Shinjuku‐ku, Tokyo 169‐8555 Japan
2. Research Institute for Materials Science and Technology Waseda University 3‐4‐1 Ohkubo, Shinjuku‐ku, Tokyo 169‐8555 Japan
3. Cooperative Major in Nuclear Energy Waseda University 3‐4‐1 Ohkubo, Shinjuku‐ku, Tokyo 169‐8555 Japan