1. Department of Physics, Beijing Normal University, Haidian District, Beijing 100875, People's Republic of China
2. School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, People's Republic of China
3. State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China
4. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, People's Republic of China