1. Department of Physics, The Hong Kong University of Science and Technology, Clear Water
Bay, Kowloon, Hong Kong China
2. National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
4. William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong China