基金项目: | The authors give their sincere thanks for the financial support from the National Science Foundation of China (No. 71003061), the Alexander von Humboldt Foundation (2007-2008), the Special Support Fund for China's Postdoctor Research Fellow (201003090) and the Humanities and the Social Sciences Project of the Ministry of Education (09YJC630225). The first author gives thanks to China Institute for Science and Technology Policy of Tsinghua University for supporting the research. |