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