1. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan
2. RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
3. Hitachi High-Technologies Corporation, Hitachinaka 312-0057, Japan
4. Research and Development Group, Hitachi, Ltd., Hatoyama 350-0395, Japan
5. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan