1. Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, 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. High Magnetic Field Laboratory, Key Laboratory of High Magnetic Field and Ion Beam Physical Biology, HFIPS, Chinese Academy of Sciences, Hefei 230031, P. R. China