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\bibitem{Helm_etal1991} {\textquotedblleft}\textit{Far-infrared spectroscopy of minibands and confined donors in GaAs/Al$_{x}$Ga$_{1-x}$As superlattices}{\textquotedblright}. Helm M, Peeters FM, DeRosa F, Colas E, Harbison JP, Florez LT, Physical review : B : condensed matter and materials physics \textbf{43}, 13983 (1991)

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