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