1. National Laboratory of Solid State Microstructures, Jiangsu Provincial Key Laboratory for Nanotechnology and Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China
2. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People's Republic of China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China