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\bibitem{Helm_etal1992} {\textquotedblleft}\textit{Infrared-spectroscopy of subbands, minibands, and donors in GaAs/AlGaAs superlattices}{\textquotedblright}. Helm M, Peeters FM, de Rosa F, Colas E, Harbison JP, Florez LT, Surface science : a journal devoted to the physics and chemistry of interfaces
T2 -- 9TH INTERNATIONAL CONF ON THE ELECTRONIC PROPERTIES OF TWO-DIMENSIONAL, SYSTEMS ( EP2DS-9 ) / 5TH INTERNATIONAL CONF ON MODULATED SEMICONDUCTOR, STRUCTURES ( MSS-5 ), JUL  \textbf{263}, 518 (1992).   http://doi.org/10.1016/0039-6028(92)90400-Z

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