1. Quantum-Phase Electronics Center and Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Correlated Electron Research Group (CERG), RIKEN-ASI, Wako, Saitama 351-0198, Japan
4. CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan