陕西省自然科学基金项目2014JQ3107;中央高校基本科研业务费专项资金项目GK201302043;陕西师范大学勤助科研创新基金项目KY2015ZD05.This work was funded by grants from the Natural Science Foundation of Shaanxi Province 2014JQ3107,Fundament Research Funds for the Central Universities GK201302043,and Shaanxi Normal University Work-Study Program For Scientific Research Innovation Fund KY2015ZD05.