1. School of Mathematics & Physics, Mianyang Teachers' College, Mianyang Normal University, Mianyang 621000, People's Republic of China
2. Department of Physics, Civil Aviation Flying Institute of China, Guanghan 618307, People's Republic of China
3. Department of Applied Physics, Chongqing University of Technology, Chongqing 400054, China
4. Department of Material Science, Sichuan University, Chengdu 610064, People's Republic of China