1. Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
2. Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA
3. Department of Physics, Oakland University, Rochester, Michigan 48309, USA
4. State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, and Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
5. Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616, USA