1. Department of Physics, Liaocheng University, Liaocheng, Shandong 252059, China
2. College of Physics and Engineering, Qufu Normal University, Qufu 273165, China
3. Department of Physics, Shanghai Jiao Tong University, Shanghai 200030, China
4. Department of Material Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China