1. School of Science, Southwest University of Science and Technology, Mianyang 621010, P. R. China
2. Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, P. R. China
3. School of Environment and Resources, Southwest University of Science and Technology, Mianyang 621010, P. R. China
4. Department of Mechanical Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong, P. R China