1. Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
2. Department of Physics, College of Liberal Arts and Sciences, National University of Defense Technology, Changsha 410073, China
3. School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Hefei 230026, China
4. State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China