1. Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA.
2. Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA.
3. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China.
4. Science Island Branch of the Graduate School, University of Science and Technology of China, Hefei 230026, China.
5. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China.