1. Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Tokyo, 113-8656, Japan
2. WPI Advanced Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan
3. Department of Physics, Tsinghua University, Beijing 100084, China
4. Correlated Electron Research Group, RIKEN, Wako 351-0198, Japan