1. School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2. MOE Key Laboratory of Advanced Materials, Tsinghua University, Beijing 100084, China
3. State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China
4. State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Frontier Science Center for Quantum Information, Tsinghua University, Beijing 100084, China
5. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
6. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China