1. School of Physics and Electronic Science, Zunyi Normal University, Zunyi 563006, P. R. China
2. College of Science, Henan University of Technology, Zhengzhou 450001, P. R. China
3. School of Science, Kaili University, Kaili, Guizhou 556011, P. R. China
4. College of Physics and Electronic Engineering, Nanning Normal University, Nanning 530001, P. R. China