1. School of Physics and Electronics, Hunan University , Changsha 410082 , China
2. Department of Physics and Astronomy, Clemson Nanomaterials Institute, Clemson University , Clemson , SC 29643 , USA
3. Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China , Hefei 230026 , China
4. School of Materials Science and Engineering, Central South University , Changsha 410083 , China
5. State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University , Changsha 410082 , China