1. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
2. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
3. School of Physics, Nanjing University, Nanjing 210093, China
4. School of Physics and Institute for Superconducting and Electronic Materials, University of Wollongong, New South Wales 2522, Australia