1. School of Mathematics and Physics, Lanzhou Jiaotong University, Lanzhou 730070, P. R. China
2. The Basic Courses Department of Lanzhou Institute of Technology, Lanzhou 730050, Gansu, P. R. China
3. College of Electrical and Information Engineering, Lanzhou University of Technology, Lanzhou 730050, P. R. China
4. School of Physics and Electromechanical Engineering, Hexi University, Zhangye 734000, P. R. China