1. Department of Physics, Yantai University, Yantai 264005, China
2. Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011, United States
3. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
4. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States