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. School of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China
4. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, P. R. China