1. Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University 1 , Beijing 100084, People's Republic of China
2. School of Physics and State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China 2 , Chengdu 610054, People's Republic of China
3. Center for Theoretical Physics, Department of Physics, Capital Normal University 3 , Beijing 100048, People's Republic of China