Appearance of new lines and change in line shape in the IR spectrum of a NaV₂O₅ single crystal at the spin-Peierls transition

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We report the first observation of new lines in the infrared spectrum of a crystal due to unit-cell doubling at the spin-Peierls transition (in NaV₂O₅). The change in the shape of the spectral lines at the spin-Peierls transition is recorded for the first time. A contour characteristic of a Fano resonance is observed above the transition temperature Tsp and the standard symmetric contour is observed below Tsp. We attribute this effect to the opening of a gap in the magnetic-excitation spectrum at the spin-Peierls transition.