1. Key Laboratory of Atomic Molecular Physics and Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, People's Republic of China
2. Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China
3. University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
4. Institute of Applied Physics and Computational Mathematics, Beijing 100088, People's Republic of China