1. Department of Physics, Liaoning Normal University, Dalian 116029, China
2. Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803-4001, USA
3. Department of Physics, School of Science, Huzhou University, Huzhou, Zhejiang 313000, China
4. Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599, USA