1. Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
2. Department of Electronics, Kyushu University, Motooka, Fukuoka 819-0395, Japan
3. Electron Microscope Facility, IBEC Innovation Platform, AIST, 16-1 Onogawa, Tsukuba 305-8569, Japan
4. Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan