1. Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics; Tsinghua University; Beijing 100084 People's Republic of China
2. Institue for Advanced Study; Tsinghua University; Beijing 100084 People's Republic of China
3. Department of Physics, McCullough Building; Stanford University; Stanford CA 94305-4045 USA
4. Department of Physics; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
5. Department of Electrical Engineering; Stanford University; Stanford CA 94305 USA
6. Collaborative Innovation Center of Quantum Matter; Tsinghua University; Beijing 100084 People's Republic of China