国家自然科学基金资助项目81200822;中央高校基本科研业务费专项资金资助项目2012jdhz27;陕西省自然科学基金资助项目No.2012JQ4004 Supported by the National Natural Science Foundation of China81200822;the Fundamental Research Funds for the Central Universities2012jdhz27;the Natural Science Foundation of Shaanxi Province2012JQ4004