1. Department
of Chemistry, Tsinghua University, Beijing 100084, China
2. Institut
für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin 12489, Germany
3. School
of Pharmaceutical Sciences, Tsinghua University, Beijing 100084, China
4. Beijing
Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
5. Shanghai
Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
6. Beijing National
Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
7. Department of Materials Science & Engineering, University of California, Los Angeles, California 90095, United States