1. State Key Laboratory for Environment-Friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China
2. School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, P. R. China
3. Condensed Matter Theory Group, Materials Theory Division, Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
4. Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden