1. Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
2. Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Tsukuba 305-8573, Japan
3. Research Center of Integrative Molecular Systems, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
4. Transmission Electron Microscopy Station, National Institute for Materials Science, Tsukuba 305-0047, Japan