1. Earthquake Research Institute, University of Tokyo 1‐1‐1 Yayoi, Bunkyo‐ku, Tokyo 113‐0032 (Japan)
2. Physics Department, University of California Riverside CA 92521 (USA)
3. Atomic Physics Laboratory, RIKEN 2‐1 Hirosawa, Wako‐shi, Saitama 351‐0198 (Japan)
4. World Premier International (WPI) Research Center International Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1‐1 Namiki, Tsukuba‐shi, Ibaraki 305‐0044 (Japan)
5. Slow Positron Facility, Institute of Materials Science Structure Science (IMSS) High Energy Accelerator Research Organization (KEK) 1‐1 Oho, Tsukuba‐shi, Ibaraki 305‐0801 (Japan)
6. Nanoceramics Center, National Institute for Materials Science (NIMS) 1‐2‐1 Sengen, Tsukuba‐shi, Ibaraki 305‐0047 (Japan)
7. Department of Applied Chemistry, Faculty of Advanced Science and Engineering Waseda University 3‐4‐1 Ohkubo, Shinjuku‐ku, Tokyo 169‐8555 (Japan)
8. Kagami Memorial Laboratory for Materials Science and Technology, Waseda University 2‐8‐26 Nishi‐Waseda, Shinjuku‐ku, Tokyo 169‐0051 (Japan)