1. State Key Laboratory of Non-linear Mechanics (LMN), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, P. R. China
2. State Key Laboratory of Non-linear Mechanics (LMN), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, P. R. China and Department of Physics, Peking University, Beijing 100871, P. R. China
3. State Key Laboratory of Non-linear Mechanics (LMN), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, P. R. China and Department of Applied Physics, Beijing University of Aeronautics and Astronautics, Beijing 100083, P. R. China