1. School
of Physics and Electronics, Hunan University, Changsha 410082, P. R. China
2. Thomas
Young Centre, University College London, London WC1E 6BT, United Kingdom
3. London
Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom
4. Department
of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
5. Department of Chemistry, University College London, London WC1E 6BT, United Kingdom
6. School
of
Physics, ICQM, and Collaborative Innovation Center of Quantum Matter, Peking University, Beijing 100871, P. R. China