国家自然科学基金资助项目30471774;教育部新世纪优秀人才支持计划资助项目NCET-05-0831;陕西省自然科学基金资助项目No.2003C1-16Supported by the National Natural Science Foundation of China30471774;the New-Century Excellent Talents Program of Minis try of EducationNCET-05-0831;the Natural Science Foundation of Shaanxi Province2003C1-16