1. 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. Songshan Lake Materials Laboratory, Dongguan 523808, P. R. China
4. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China