1. Department of Physics and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China
2. Key Laboratory of Computational Physical Sciences (Ministry of Education), Fudan University, Shanghai 200433, China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
4. Department of Physics, University of California, Davis, California 95616, USA
5. Department of Physics, University of California, Berkeley, California 94720, USA
6. Physics Department, Georgetown University, Washington, DC 20057, USA
7. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA