1. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States
2. U.S. DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States
3. National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China