1. Department of Physics, Qufu Normal University, Qufu 273165, P. R. China
2. Department of Physics, Dalian University of Technology, Dalian 116024, P. R. China
3. School of Physics and Electronic Engineering, Linyi University, Linyi 276005, P. R. China
4. Department of Applied Physics, University of Extremadura, Badajoz 06006, Spain