1. Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China
2. Institute of Theoretical Physics and Department of Physics, Shanxi University, Taiyuan, Shanxi 030006, P.R. China
3. Department of Physics, Yanbei Normal Institute, Datong, Shanxi 037000, P.R. China
4. Department of Physics, University of Kaiserslautern, D-67653, Kaiserslautern, Germany
5. Department of Physics, Guangzhou Normal College, Guangzhou, Guangdong 510400, P.R. China