1. Professor, Dept. of Physics, Anqing Teachers College, Anqing Anhui 246011, P.R. China (corresponding author).
2. Professor, State Key Lab of Turbulence and Complex System, College of Engineering, Peking Univ., Beijing 100871, P.R. China.
3. Dept. of Physics, Anqing Teachers College, Anqing Anhui 246011, P.R. China.
4. Associate Professor, Dept. of Physics, Anqing Teachers College, Anqing Anhui 246011, P.R. China.