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\bibitem{Helm_etal2000} {\textquotedblleft}\textit{Interminiband spectroscopy of biased superlattices}{\textquotedblright}. Helm M, Hilber W, Strasser G, de Meester R, Peeters FM, Wacker A, Physica. E: Low-dimensional systems and nanostructures \textbf{7}, 274 (2000).   http://doi.org/10.1016/S1386-9477(99)00275-1

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