1. State Key Laboratory of Superlattices and Microstructures; Institute of Semiconductors, Chinese Academy of Sciences; Beijing 100083 P. R. China
2. College of Materials Science and Opto-Electronic Technology; University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. School of Physics and Astronomy; University of Nottingham; Nottingham NG7 2RD UK
4. CAS Center for Excellence in Topological Quantum Computation; University of Chinese Academy of Sciences; Beijing 100049 P. R. China