1. Department
of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, People’s Republic of China
2. School
of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China
3. Department
of Mechanical Engineering and Materials Science, Department of Chemistry, and the Smalley
Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States
4. Collaborative
Innovation Center of Quantum Matter, Beijing 100084, People’s Republic of China
5. Institute
for Advanced Study, Tsinghua University, Beijing 100084, People’s Republic of China