1. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
2. Key Laboratory of Materials Physics, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China
3. Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
4. State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China
5. Beijing Superstring Academy of Memory Technology, Beijing 100176, China
6. Chongqing Research Institute, National University of Singapore, Chongqing 401120, China