1. Hefei
National Laboratory for Physical Science at Microscale and Department
of Physics, University of Science and Technology of China, Hefei 230026, China
2. Key
Laboratory of Strongly-Coupled Quantum Matter Physics, University
of Science and Technology of China, Chinese Academy of Sciences, Hefei 230026, China
3. Collaborative
Innovation Center of Advanced Microstructures, Nanjing 210093, China
4. Cambridge
Graphene Centre, University of Cambridge, Cambridge CB3 0FA, United Kingdom