1. School of Physics and Information Engineering, Jiangsu Second Normal University, Nanjing 211200, P. R. China
2. Herbert Gleiter Institute of Nanoscience, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
3. Department of Physics, School of Physics and Materials Science, Nanchang University, Nanchang 330033, P. R. China