1. Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P.R. China
2. Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, P. R. China
3. High Magnetic Field Laboratory, Chinese Academy of Science, Hefei 230031, P. R. China
4. University of Science and Technology of China, Hefei 230026, P.R. China
5. Applied Physics and Applied Mathematics Department, Fu Foundation School of Engineering and Applied Science, Columbia University, New York, NY 10027, USA
6. Key Laboratory of Optic-Electronic Information and Materials of Hebei Province, Institute of Life Science and Green Development, College of Physics Science and Technology, Hebei University, Baoding 071002, China
7. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China