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\bibitem{Helm_etal1994} {\textquotedblleft}\textit{Bloch and localized electrons in semiconductor superlattices}{\textquotedblright}. Helm M, Hilber W, Fromherz T, Peeters FM, Alavi K, Pathak RN, Semiconductor science and technology \textbf{9}, 1989 (1994).   http://doi.org/10.1088/0268-1242/9/11S/022

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