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\bibitem{Helm_etal1993} {\textquotedblleft}\textit{Infrared absorption in superlattices: a probe of the miniband dispersion and the structure of the impurity band}{\textquotedblright}. Helm M, Hilber W, Fromherz T, Peeters FM, Alavi K, Pathak R, Physical review : B : condensed matter and materials physics \textbf{48}, 1601 (1993).   http://doi.org/10.1103/PhysRevB.48.1601

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