基金Supported by the National Natural Science Foundation of China under Grant Nos 10434100 and 10774122, the Foundation of China/Ireland Science and Technology Collaboration Research under Grant No CI-2004-07, the Specialized Research Fund for the Doctoral Programme of Higher Education of China under Grant No 20070736001, and the Foundation of Northwest Normal University under Grant No NWNU-KJCXGC-03-21.