1. Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
2. Department of Physics, COMSATS Institute of Information and Technology, Lahore 54000, Pakistan
3. Institute of Physics, University of Chinese Academy of Sciences, Beijing 100190, P. R. China
4. Department of Materials Science and Engineering College of Engineering, Peking University Beijing 100871, P. R. China