1. Department of Physics and Astronomy, University of California, Riverside, California 92521, United States
2. International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, P. R. China
3. National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
4. Department of Physics and Astronomy, University of California, Irvine, California 92697, United States