国家自然科学基金(批准号:60607006,61079042);教育部留学回国基金(批准号:20081667);湖北省自然科学基金(批准号:2007ABA058)资助的课题~~supported by the National Natural Science Foundation of China(Grant No.60607006,61079042);the Science Research Foundation of Ministry of Education for Returned Overseas Scholar,China(Grant No.20081667);the Hubei Natural Science Foundation of China(Grant No. 2007ABA058)