1. School of Physics, Southeast University, Nanjing 211189, China
2. School of Physics and Telecommunication Engineering, Zhoukou Normal University, Zhoukou 466001, China
3. Department of Materials and Nanophysics, School of Engineering Sciences, KTH Royal Institute of Technology, 16440 Kista, Sweden
4. National Laboratory of Solid Microstructures, Nanjing University, Nanjing 210093, China