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\bibitem{Helm_etal1999} {\textquotedblleft}\textit{Simultaneous investigation of vertical transport and intersubband absorption in a superlattice: continuum Wannier-Strak ladders and next-nearest neighbor tunneling}{\textquotedblright}. Helm M, Hilber W, Strasser G, de Meester R, Peeters FM, Wacker A, Physica: B : condensed matter \textbf{272}, 194 (1999).   http://doi.org/10.1016/S0921-4526(99)00269-0

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