国家自然科学基金50807018;教育部全国百篇优秀博士论文作者专项基金200541;教育部博士点基金新教师项目200805611056.Project Supported by National Natural Science Foundation of China50807018;Foundation for the Author of National Excellent Doctoral Dissertation of PR China200541;Doctoral Fund of Ministry of Education of China200805611056