1. Department of Applied Physics; Nanjing Tech University; Nanjing 210009 PR China
2. National Laboratory of Solid State Microstructures; Nanjing University; Nanjing 210093 PR China
3. College of Science, Nanjing University of Aeronautics and Astronautics; Nanjing 210016 PR China
4. Physics Department; Southeast University; Nanjing 211189 PR China