1. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
2. Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, Beijing 100083, China
3. Institute of Microelectronics, Peking University, Beijing 100871, China
4. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
5. Department of Physics, Tsinghua University, Beijing 100084, China