1. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, People's Republic of China
2. Nano-Optoelectronics Frontier Center of Ministry of Education, Peking University, Beijing 100871, People's Republic of China
3. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People's Republic of China
4. Collaborative Innovation Center of Quantum Matter, Beijing 100871, People's Republic of China