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\bibitem{CostaFilho_etal2007} {\textquotedblleft}\textit{Graphene ribbons with a line of impurities: oOpening of a gap}{\textquotedblright}. Costa Filho RN, Farias GA, Peeters FM, Physical review : B : condensed matter and materials physics \textbf{76}, 193409 (2007).   http://doi.org/10.1103/PhysRevB.76.193409

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