1. Department of Physics, Southeast University, Nanjing 211189, China
2. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China
3. Department of Materials Science and Engineering, Nanjing University, Nanjing 210008, China
4. College of Physics, Qingdao University, Qingdao 266071, China