1. Key laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, 230031 Hefei, P. R. China
2. Science Island Branch of Graduate School, University of Science and Technology of China, 230026 Hefei, P. R. China
3. Institute for Computational Materials Science, School of Physics and Electronics, Henan University, 475004 Kaifeng, P. R. China