1. State Key Laboratory of Low Dimensional Quantum Physics Department of Physics Tsinghua University Beijing 100084 P. R. China
2. College of Optoelectronic Science and Engineering National University of Defense Technology Changsha 410073 P. R. China
3. Collaborative Innovation Center of Quantum Matter Beijing 100084 P. R. China
4. RIKEN Center for Emergent Matter Science (CEMS) Wako Saitama 351‐0198 Japan
5. Department of Physics McCullough Building Stanford University Stanford CA 94305‐4045 USA