1. Key Laboratory for Renewable Energy Chinese Academy of Sciences (CAS) Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Chinese Academy of Sciences Beijing 100190 P. R. China
2. School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Huairou Division Institute of Physics Chinese Academy of Sciences Beijing 101400 P. R. China
4. State Key Laboratory of Superhard Materials College of Physics Jilin University Changchun 130012 P. R. China
5. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China
6. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China