1. Collaborative Innovation Center of Advanced Microstructures, Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China
2. Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
3. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
4. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China