1. Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan.
2. California NanoSystems Institute, University of California at Santa Barbara, Santa Barbara CA 93106, USA.
3. RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
4. Central Research Institute of Electric Power Industry (CRIEPI), Kanagawa 240-0196, Japan.
5. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.