1. School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China
2. Key Laboratory of Computational Physical Sciences (Ministry of Education), State Key Laboratory of Surface Physics, and Department of Physics, Fudan University, Shanghai 200433, China
3. Shanghai Qi Zhi Institute, Shanghai 200030, China
4. Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States