1. National Synchrotron Radiation Laboratory; University of Science and Technology of China; Hefei 230029 P. R. China
2. Institute of Physics; Chinese Academy of Sciences; Beijing 100190 P. R. China
3. Shanghai Synchrotron Radiation Facilities; Shanghai Institute of Applied Physics Chinese Academy of Science; Shanghai 201204 P. R. China
4. Hefei National Laboratory for Physical Science at the Microscale; School of Chemistry and Materials Science, University of Science and Technology of China; Hefei, Anhui 230026 P. R. China