1. Department of Physics Beijing Normal University Beijing 100875 China
2. School of Materials Science and Engineering Tsinghua University Beijing 100084 China
3. Advanced Research Institute of Multidisciplinary Science Beijing Institute of Technology Beijing 100081 China
4. Institute of Physics Chinese Academy of Science Beijing 100190 China
5. Faculty of Information Technology Beijing University of Technology Beijing 100124 China
6. Department of Physics and Astronomy University of Manchester Oxford Rd Manchester M13 9PL UK