1. National Center
for Electron Microscopy in Beijing, School of Materials Science and
Engineering, The State Key Laboratory of New Ceramics and Fine Processing,
Key Laboratory of Advanced Materials, Tsinghua University, Beijing 100084, China
2. Beijing National
Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. Department of Physics, Southeast University, Nanjing 211189, China