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\bibitem{Gershenzon_etal1987} Gershenzon, E. M., Gol{\textquoteright}tsman, G. N., Karasik, B. S., \& Semenov, A. D. (1987). Measurement of the energy gap in the compound YBaCu$_{3}$O$_{9-δ}$ on the basis of the IR absorption spectrum. \textit{JETP Lett.}, \textit{46}(5), 237--238.

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