基金Supported by the National Natural Science Foundation of China under Grant No.60434030(国家自然科学基金)the National High-Tech Research and Development Plan of China under Grant No.2006AA01Z218(国家高技术研究发展计划(863))the National Research Foundation for the Doctoral Program of Higher Education of China under Grant No.20050335020(国家教育部博士点基金)