## Acknowledgments 

We acknowledge the valuable discussions with C. X. Liu, X. L. Qi, Q. Niu, S. Q. Shen, and the supports from the NSF of China, the 973 Program of China (No. 2007CB925000 and 2010CB923000), and the International Science and Technology Cooperation Program of China. SCZ is supported by the US NSF under grant numbers DMR-0904264. 

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