国家自然科学基金61308107;高等学校博士学科点专项科研基金20126118120003;陕西省自然科学基金批准号:2012JQ5002资助的课题.@@@@* Project supported by the National Natural Science Foundation of ChinaGrant 61308107;the Specialized Research Fund for the Doctoral Program of Higher Education of ChinaGrant 20126118120003;the Natural Science Foundation of Shanxi Province, ChinaGrant 2012JQ5002