国家自然科学基金资助项目81273211,81302527;中国博士后科学基金资助项目2013M542356;陕西省国际合作重点项目2013KW21Supported by the National Natural Science Foundation of China81273211,81302527;China Postdoctoral Science Foundation2013M542356;International Cooperation Foundation of Shaanxi Province2013KW21