1. College
of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
2. Beijing
National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
3. Beijing
Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
4. Department
of Materials, Imperial College London, Royal School of Mines, Exhibition
Road, London SW7 2AZ, U.K.