1. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. JST, PRESTO, Kawaguchi, Saitama 332-0012, Japan
3. Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
4. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
5. RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan