1. School of Physics, Southeast University, Nanjing 211189, 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. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
4. Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA
5. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA