1. School of Physics and Physical Engineering, Qufu Normal University, Qufu 273165, China
2. Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
4. Condensed Matter Theory Group, Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden