1. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China
2. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, People's Republic of China
3. Physics Department, The Hongkong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
4. HKUST Shenzhen Research Institute, Shenzhen 518057, People's Republic of China