1. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
2. National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, and School of Physics, Nanjing University, Nanjing 210093, China
3. School of Physics, Nankai University, Tianjin 300071, China
4. Department of Electrical and Computer Engineering, University of California, Riverside, California 92521, United States
5. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
6. Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
7. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
8. Department of Physics and Astronomy, University of California, Riverside, California 92521, United States