1. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan;
2. Department of Physics, Durham University, Durham DH1 3LE, United Kingdom;
3. State Key Laboratory of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, P.R. China;
4. Collaborative Innovation Center of Quantum Matter, Beijing 100084, P.R. China
5. Institute of Physics, Academia Sinica, Taipei 105, Taiwan