1. Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA.
2. Department of Physics, University of Washington, Seattle, WA 98195, USA.
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. Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA.