1. State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
2. Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
3. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
4. Institute of Superlubricity Technology, Research Institute of Tsinghua University in Shenzhen, Shenzhen 518000, China
5. Department of Physics, Zhejiang University, Hangzhou 310027, China