1. Mechanical Engineering Department of Qinghai University, Xining 810016, P. R. China
2. Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, P. R. China
3. Department of Basic Education, Qinghai University, Xining 810016, P. R. China
4. School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan 430074, P. R. China
5. Department of Physics and Astronomy, University of Nevada, Las Vegas, Nevada 89154, United States