1. Beijing National Laboratory for Condensed Matter Physics and 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. Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450001, P. R. China
4. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, P. R. China