1. State Key Laboratory
of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
2. Beijing National Laboratory for Condensed Matter Physics, Institute
of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. Department of Physics, Stanford University, McCullough Building, Stanford, California 94305-4045, United States
4. Collaborative Innovation Center of Quantum Matter, Beijing 100084, China