1. State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
2. Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China
3. School of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China