1. School of Physics and Electronic Information, Huaibei Normal University, Huaibei, Anhui 235000, P. R. China
2. Center of Modern Experiment and Technology, Anhui University, Hefei, Anhui 230601, P. R. China
3. College of Physics and Energy, Fujian Normal University, Fuzhou, Fujian 350007, P. R. China
4. High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China