基金Supported by the National High-Tech Research and Development Plan of China under Grant No.2007AA04Z148 (国家高技术研究发 展计划(863))the National Natural Science Foundation of China under Grant No.60573126 (国家自然科学基金)the National Basic Research Program of China under Grant No.2002CB312005 (国家重点基础研究发展计划(973))