1. Institute of Materials, China Academy of Engineering Physics, 621908, Jiangyou, Sichuan, P. R. China
2. School of Physics, University of Electronic Science and Technology of China, 611731, Chengdu, Sichuan, China
3. State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, 100084, Beijing, P. R. China