1. Department of Physics, California Institute of Technology, Pasadena, California 91125, United States
2. Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States
3. Kavli Nanoscience
Institute, California Institute of Technology, Pasadena, California 91125, United States
4. Beijing National Laboratory for Condensed Matter Physics, Institute
of Physics, The Chinese Academy of Sciences, Beijing 100190, China
5. Department of Physics, University of California−San Diego, La Jolla, California 92093, United States
6. Department
of Physics, Columbia University, New York, New York 10027, United States
7. State Key Laboratory of Low-Dimensional Quantum Physics, Department
of Physics, Tsinghua University, Beijing 100084, China