1. Research Center of Materials Science; School of Materials Science and Engineering; Beijing Institute of Technology; Beijing 100081 P.R. China
2. Department of Physics; Beihang University; Beijing 100191 P.R. China
3. Beijing National Laboratory for Molecular Science; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 P.R. China
4. Shanghai Synchrotron Radiation Facility; Shanghai Institute of Applied Physics; Shanghai 201800 P.R. China
5. Department of Materials Physics and Chemistry; University of Science and Technology Beijing; Beijing 100083 P.R. China
6. Institute for Superconducting and Electronic Materials; University of Wollongong; Wollongong NSW 2500 Australia