1. Department of Applied Physics and Quantum Phase Electronics Center, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan.
2. RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan.
3. Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan.
4. PRESTO, Japan Science and Technology Agency, Chiyoda-ku, Tokyo 102-0075, Japan.