1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
2. Key Laboratory of Spintronics Materials, Devices and Systems of Zhejiang Province, Hangzhou 311300, China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
4. Key Laboratory for the Physics and Chemistry of Nanodevices, Center for Carbon-Based Electronics, School of Electronics, Peking University, Beijing 100871, China