1. School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, P. R. China
2. Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
3. School of Science, Anhui University of Science and Technology, Huainan 232001, P. R. China
4. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China