1. School of Physics and Collaborative Innovation Centre of Quantum Matter, Peking University, Beijing, People's Republic of China
2. Thomas Young Centre, London Centre for Nanotechnology, and Department of Physics and Astronomy, University College London, London, UK
3. Laboratory of Physical Chemistry, ETH Zurich, Zurich, Switzerland
4. Department of Electronic Structure Theory, Max Plank Institute for Solid State Research, Stuttgart, Germany
5. School of Physics and Electronics, Hunan University, Changsha, People's Republic of China
6. State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Peking University, Beijing, People's Republic of China