1. Department of Physics, Harbin Institute of Technology, Harbin 150080, China
2. Condensed Matter Science and Technology Institute, The School of Instrumentation Science and Engineering, Harbin Institute of Technology, Harbin 150080, China
3. School of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China
4. Department of Mathematics and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA