1. College of Materials Science and Opto-Electronic Technology & Center of Materials Science and Optoelectronics Engineering & CAS Center for Excellence in Topological Quantum Computation & CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
2. Hebei Key Laboratory of Optic-Electronic Information and Materials, the College of Physics Science and Technology, Hebei University, Baoding 071002, China
3. National Center for Nanoscience and Technology, Beijing 100190, China
4. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China