基金The project partly supported by National Natural Science Foundation of China under Grant Nos. 10225525 and 10435080 and Knowledge Innovation Project of the Chinese Academy of Sciences under Grant No. KJCX2-SW-N02. We thank H.C. Chiang, G.M. Jin, X.G. Li, J.Y. Liu, P.N. Shen, J.J. Xie, H.S. Xu, and W.L. Zhan for useful discussions.