1. Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
2. Electron Microscope Facility, TIA, AIST, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
3. PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan