1. International Center for Quantum Materials; School of Physics; Peking University; Beijing 100871 China
2. Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems; School of Physics; Beijing Institute of Technology; Beijing 100081 China
3. State Key Laboratory of Low Dimensional Quantum Physics; Department of Physics; Tsinghua University; Beijing 100084 China
4. Tsinghua-Foxconn Nanotechnology Research Center; Tsinghua University; Beijing 100084 China
5. Collaborative Innovation Center of Quantum Matter; Beijing 100871 China