1. College of Science, Guilin University of Technology, Guilin 541004, P. R. China
2. School of Physics, Sun Yat-Sen University, Guangzhou, 510275, P. R. China
3. Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China
4. School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China
5. MOE Key Laboratory of New Processing Technology for Nonferrous Metal and Materials, Key Laboratory of Low-dimensional Structural Physics and Application, Education Department of Guangxi Zhuang Autonomous Region, Guilin University of Technology, Guilin 541004, P. R. China