1. Materials Science and Engineering Shanghai University Shanghai 200072 P. R. China
2. Materials Science and Engineering Guilin University of Technology Guilin 541000 P. R. China
3. Yanshan University Qinhuangdao 006004 P. R. China
4. Institute of High Energy Physics Chinese Academy of Science Beijing 100049 P. R. China
5. Guilin University of Electronic Technology Guilin 541004 P. R. China