1. Tianjin Key Laboratory of Low-Dimensional Materials
2. Physics and Preparing Technology
3. Faculty of Science
4. Tianjin University
5. Tianjin 300072, P. R. China
6. College of Physics and Materials Science
7. Tianjin Normal University
8. Tianjin 300074, P. R. China
9. Department of Physics
10. University of Hong Kong
11. Hong Kong, P. R. China
12. Beijing National Laboratory for Condensed Matter Physics
13. Institute of Physics
14. Chinese Academy of Sciences
15. Beijing 100190, P. R. China
16. National Center for Nanoscience and Technology of China