基金This paper was financially supported by the National Basic Research Program of China （973 Program, Grant No. 2009CB219507）, the National Natural Science Foundation of China （Grant Nos. 50639030 and 50979070）, and the National Science and Technology Major Project of China （Grant Nos. 2011ZX05056-002 and 2011ZX05026-004）. Acknowledgements - The authors would like to thank Mr. ZHAO Yanbing, Ms. HU Cun, Mr. LIU Chenglin, Mr. HUANG Wei and Mr. YUE Yanzhao, for their generous help with the experiments and calculations.