1. National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
2. National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China
3. Department of Physics, Southeast University, Nanjing 211189, China
4. College of Engineering and Applied Science, Nanjing University, Nanjing 210093, China
5. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong