1. School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 61003, P. R. China
2. Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu 61003, P. R. China
3. College of Physics and Energy, Fujian Normal University, Fuzhou 350117, P. R. China