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))