1. School of Physics and Technology, University of Jinan, Jinan 250022, Shandong Province, P. R. China
2. School of Mathematics and Physics, Anhui University of Technology, Ma’anshan 243032, Anhui Province, P. R. China
3. Shandong Labor Vocational and Technical College, Jinan 250300, Shandong Province, P. R. China