1. Department of Physics, Zhejiang University, Hangzhou 310012, P. R. China
2. School of Civil Engineering, Zhengzhou Institute of Technology, Zhengzhou 450044, P. R. China
3. Institute of Applied Mechanics, Zhejiang University, Hangzhou 310012, P. R. China
4. Department of Physics, Jiangsu University, Zhenjiang 212013, P. R. China