1. School of Physics and Electronics Hunan University Changsha 410082 P. R. China
2. Department of Physics and Astronomy Clemson Nanomaterials Institute Clemson University Clemson SC 29634 USA
3. School of Materials Science and Engineering Central South University Changsha 410083 P. R. China
4. State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Hunan University Changsha 410082 P. R. China