国家自然科学基金51177140;中央高校基本科研业务费2682013ZT20;2013年西南交通大学优秀博士学位论文培育项目资助的课题.@@@@* Project supported by the National Natural Science Foundation of ChinaGrant 51177140;the Fundamental Research Fund for the Central Universities, ChinaGrant 2682013ZT20;the 2013 Cultivation Program for the Excellent Doctoral Dissertation of Southwest Jiaotong University, China