基金Author to whom correspondence should be addressed. E-mail: zcdong@ustc.edu.cn, FAX: +86-551-3600103 This work was supported by the National Basic Research Program of China (No.2006CB922003 and No.2011CB921402), the Chinese Academy of Sciences (No.KJCX2.YW.H06), and the National Natural Science Foundation of China (No.91021004, No.10574117, and No.10974186).