1. School of Science Chongqing University of Technology Chongqing 400054 P. R. China
2. Ningbo Institute of Materials Technology & Engineering Chinese Academy of Science Ningbo 315201 P. R. China
3. Institute of Materials Ningbo University of Technology Ningbo 315016 P. R. China
4. Department of Mathematics and Physics Chongqing University of Posts and Telecommunications Chongqing 400065 P. R. China
5. Department of Physics Georgia Southern University Statesboro GA 30460 USA