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