1. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), the University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. Institute for Solid State Physics (ISSP), the University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan
4. Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan