1. School of Materials Science and Engineering, Tsinghua University , Beijing , China
2. State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University , Beijing , China
3. School of Nuclear Science and Technology, Xi'an Jiaotong University , Xi'an , China
4. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences , Beijing , China
5. School of Physical Sciences, University of Chinese Academy of Sciences , Beijing , China