1. Department of Applied Physics, Lanzhou University of Technology, Lanzhou, Gansu 730050, China
2. School of Mathematics and Physics, Lanzhou Jiaotong University, Lanzhou 730070, China
3. School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, Gansu 730050, China
4. College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China