1. Wuhan National Laboratory for OptoelectronicsHuazhong University of Science and Technology Wuhan 430074 P. R. China
2. Department of Physics and Texas Center for Superconductivity at the University of Houston (TcSUH)University of Houston Houston TX 77204 USA
3. College of ScienceHuazhong Agricultural University Wuhan 430070 P. R. China