1. RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
2. Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan
3. Transmission Electron Microscopy Station and Surface Physics and Structure Unit, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
4. Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan