1. School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou510006, China
2. Department of Physics, Iowa State University, Ames, Iowa50011, United States
3. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York10027, United States
4. Ames Laboratory, U.S. Department of Energy, Ames, Iowa50011, United States
5. Department of Physics, Xiamen University, Xiamen361005, China