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\bibitem{Helm_etal1994} {\textquotedblleft}\textit{Mini-band dispersion, critical points, and impurity bands in superlattices: an infrared absorption study}{\textquotedblright}. Helm M, Hilber W, Fromherz T, Peeters FM, Alavi K, Pathak RN, Solid state electronics \textbf{37}, 1277 (1994).   http://doi.org/10.1016/0038-1101(94)90407-3

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