1. Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, P.R. China
2. Mathematics and Physics Department, Luoyang Institute of Science and Technology, Luoyang 471023, P.R. China
3. Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, P.R. China