1. Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, Advanced Science Research Laboratory, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, 369-0293, Japan, and High Voltage Electron Microscopy Station, National Institute for Materials Science, 3-13 Sakura, Tsukuba, 305-0003, Japan