1. State
Key Laboratory of Artificial Microstructure and Mesoscopic Physics,
School of Physics, Peking University, Beijing 100871, People’s Republic of China
2. Collaborative Innovation Center of Quantum Matter, Beijing 100871, People’s Republic of China
3. Laboratory
of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing 100083, People’s Republic of China
4. Department
of Physics, Tsinghua University, Beijing 100084, People’s Republic of China