L.H. thanks Professor A. Rycerz for helpful discussions. This work was supported by NSF of China under Grants No. 11135001, No. 11375074, and No. 11422541, as well as by the Doctoral Fund of Ministry of Education of China under Grant No. 20130211110008. Y.C.L. and H.Y.X. were supported by AFOSR under Grant No. FA9550-15-1-0151 and by ONR under Grant No. N00014-15-1-2405.
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