1. Northeastern University, Shenyang 110819, P.R.China
2. Harbin Institute of Technology, Harbin 150001, P.R.China
3. Post Box 458, Harbin Institute of Technology, Harbin 150001, P.R.China. huqinghua@hit.edu.cn
4. Post Box 458, Harbin Institute of Technology, Harbin 150001, P.R.China