1. National Laboratory of Solid State Microstructures, School
of Physics, Nanjing University, Nanjing 210093, P. R. China
2. Collaborative Innovation Center of Advanced
Microstructures, Nanjing University, Nanjing 210093, P. R. China
3. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States
4. Ames Laboratory—U.S. Department of Energy, Iowa State University, Ames, Iowa 50011, United States