1. International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan
2. Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan
3. Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0814, Japan
4. Electron Microscopy Analysis Station, NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan