1. Tsinghua-Foxconn Nanotechnology Research Center Tsinghua University Beijing 100084 China
2. Laboratory for Advanced Interfacial Materials and Devices School of Fashion and Textiles The Hong Kong Polytechnic University Hong Kong SAR 99077 China
3. Department of Physics Tsinghua University Beijing 100084 China
4. Laboratory of Computational Physics Institute of Applied Physics and Computational Mathematics Beijing 100088 China
5. School of Materials Science and Technology China University of Geosciences Beijing 100083 China
6. Frontier Science Center for Quantum Information Tsinghua University Beijing 100084 China
7. State Key Laboratory of Low-Dimensional Quantum Physics Tsinghua University Beijing 100084 China