tokens: This work was supported by the National Natural Science Foundation of China (NSFC) under the grant Nos. 10878002, 11025314, 10673004, 10610099a, and 11533005, as well as NKBRSF under the grants 2011CB811402 and 2014CB744203. W.C. acknowledges the support of the US NSF (AGS-0847126 and AGS-1250818) and NASA (NNX13AG14G).
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