1. College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
2. State Key Laboratory of Surface Physics and Key Laboratory for Computational Physical Sciences (MOE) & Department of Physics, Fudan University, Shanghai 200433, China
3. College of Physics, Optoelectronics and Energy, Soochow University, Suzhou, Jiangsu 215006, China
4. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
5. Institute of Materials Science, Technical University of Darmstadt, Darmstadt 64287, Germany
6. Department of Physics and Astronomy, University of California, Irvine, Irvine, California 92697-4575, United States