1. Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, P. R. China
2. College of Physics, Sichuan University, Chengdu 610065, P. R. China
3. Research Institute of Frontier Science, Southwest Jiaotong University, Chengdu 610031, P. R. China