本文由国家基础研究发展计划项目2013CB429903、甘肃省青年科技基金计划项目145RJYK278和陇原青年创新人才扶持计划项目资助;This work was supported by the National Basic Research Programs of China 2013CB429903,the Gansu Province Youth Science and Technology Research Fund 145RJYK278,and the Young Creative Talents Fostering Program of Gansu Province.