1. School of Mathematics and Physics, Southwest University of Science and Technology, Mianyang 621010, P. R. China
2. School of Materials and Chemistry, Southwest University of Science and Technology, Mianyang 621010, P. R. China
3. Key Laboratory for Neutron Physics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, P. R. China
4. Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, P. R. China
5. Key Laboratory of Transparent Opto-Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, P. R. China