1. Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China
3. Advanced Nanofabrication, Imaging and Characterization Core Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 239955, Kingdom of Saudi Arabia
4. Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 239955, Kingdom of Saudi Arabia