1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
4. State Key Laboratory of Surface Physics, Department of Physics, and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China
5. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China