1. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, PR China
2. Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, PR China
3. Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
4. Institute of Superlubricity Technology, Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, PR China