1. Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China
2. Collaborative Innovation Center of Quantum Matter, Beijing 100084, China
3. Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78758, USA
4. Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan