1. State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics & Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China
2. State Key Laboratory of New Ceramics and Fine Processing, School
of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
3. Collaborative Innovation Center
of Quantum Matter, Beijing 100084, China