基金Acknowledgments We thank Drs Fengyong Liu and Sheng Luan at UC Berkeley, USA, for their discussion and help with the writing of the manuscript. This work was supported by grants from the National Natural Science Foundation of China (no. 30225037, 30471991, 30570731), National Basic Research Program of China (973 Program) (no. 2006CB503909, 2004CB518603), the "111" Project, and the Natural Science Foundation of Jiangsu Province (no. BK2004082, BK2006714).