1. National Laboratory of Solid State Microstructures; School of Physics and Collaborative Innovation; Center of Advanced Microstructures; Nanjing University; Nanjing 210093 P. R. China
2. Department of Materials Science and Engineering; National Chiao Tung University; Hsinchu 30010 Taiwan
3. Department of Electrophysics; National Chiao Tung University; Hsinchu 30010 Taiwan
4. Advanced Light Source; Lawrence Berkeley National Laboratory; Berkeley CA 94720 USA
5. Department of Materials and Engineering; Pennsylvania State University; University Park PA 16802 USA
6. Department of Physics; Durham University; Durham DH1 3LE UK
7. State Key Laboratory of Low-Dimensional Quantum Physics; Department of Physics; Tsinghua University; Beijing 100084 China
8. Collaborative Innovation Center of Quantum Matter; Beijing 100084 China