1. Department of Physics, Southeast University, Nanjing 210096, P. R. China
2. Institute of Theoretical Physics, the Chinese Academy of Sciences, Beijing 100080, China
3. Department of Physics, Nanjing normal University, Nanjing 210097, P. R. China
4. Center of Theoretical Physics, Nanjing University, Nanjing 210093, P. R. China
5. Department of Physics, Sichuan Union University, Chengdu 610064, China