Supported by the National Natural Science Foundation of China under Grant Nos.90304010; 60673164(国家自然科学基金);the Program for New Century Excellent Talents in University of China under Grant No.NCET050683(新世纪优秀人才支持计划);the National Research Foundation for the Doctoral Program of Ministry of Education of China under Grant No.20060533057(国家教育部博士点基金);the Provincial Natural Science Foundation of Hu’nan of China under Grant No.06JJ10009(湖南省杰出青年基金);the Scientific Research Fund of Hu’nan Provincial Education Department of China under Grant No.05D054(湖南省教育厅资助科研项目)