1. State Key Laboratory of Artificial Microstructure and Mesoscopic PhysicsSchool of PhysicsPeking University Beijing 100871 P. R. China
2. Electron Microscopy LaboratorySchool of PhysicsPeking University Beijing 100871 P. R. China
3. Collaborative Innovation Center of Quantum Matter Beijing 100871 P. R. China
4. Nano‐optoelectronics Frontier Center of Ministry of Education (NFC‐MOE)Peking University Beijing 100871 P. R. China