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