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. School of Physics and Materials Science Anhui University Hefei 230601 P. R. China
4. High Magnetic Field Laboratory Chinese Academy of Sciences Hefei 230031 P. R. China