1. Department of Applied Physics, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan
3. Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
4. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan