The National Natural Science Foundation of China30700088,31200361;funds from the South-Central University for NationalitiesYZZ08007,XJS09002;the special fund for basic scientific research of central collegesCZY11008;国家自然科学基金项目30700088,31200361;中南民族大学校基金项目YZZ08007,XJS09002;中南民族大学中央高校基金项目CZY11008