1. Department of Basic Science, University of Tokyo, Bunkyo, Tokyo 115-8902, Japan
2. PRESTO, Japan Science and Technology Agency, Chiyoda, Tokyo 102-0076, Japan
3. Department of Physics, Tohoku University, Sendai 980-8578, Japan
4. Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, U.S.A.
5. RIKEN, Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan
6. Quantum-Phase Electronics Center and Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan