1. Department of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen, Guangdong 518055, P.R. China
2. Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204, United States
3. Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, United States
4. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
5. Science Island Branch of the Graduate School, University of Science and Technology of China, Hefei 230026, China
6. School of Science, Harbin Institute of Technology, Shenzhen, Guangdong 518055, P.R. China