1. LCP, Institute of Applied Physics and Computational Mathematics, Beijing 100094, P. R. China
2. Institute of Materials, China Academy of Engineering and Physics, Mailbox No. 9-21, Huafengxincun, Jiangyou 621908, Sichuan, P. R. China
3. National Collaborative Innovation Center for Nuclear Waste and Environmental Safety, Southwest University of Science and Technology, Mianyang 621010, P. R. China
4. Materials Theory Division, Department of Physics and Astronomy, Condensed Matter Theory Group, Uppsala SE-751 21, Sweden
5. Center for Applied Physics and Technology, Peking University, Beijing 100871, P. R. China
6. China Academy of Engineering and Physics, Mianyang 621900, Sichuan, P. R. China
7. Physics Department, Tsinghua University, Beijing 100001, P. R. China