1. Key Laboratory of Materials Physics Institute of Solid State Physics HFIPS Chinese Academy of Sciences Hefei 230031 P. R. China
2. University of Science and Technology of China Hefei 230026 P. R. China
3. College of Physics and Electronic Information Inner Mongolia Normal University Hohhot 010000 P. R. China
4. High Magnetic Field Laboratory Chinese Academy of Sciences Hefei 230031 P. R. China