1. Kagami Memorial Laboratory for Material Science and Technology, Waseda University, 2-8-26 Nish-waseda, Shinjuku-ku, Tokyo 169-0051, Japan
2. Department of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169, Japan
3. JEOL Limited, 1-2 Musashino 3-chome, Akishima, Tokyo 196-8558, Japan
4. Kagami Memorial Laboratory for Material Science and Technology, Waseda University, 2-8-26 Nish-waseda, Shinjuku-ku, Tokyo 169-0051, Japan and Department of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169, Japan