Fano resonances in the infrared spectra of phonons in hyperkagome Na 3 Ir 3 O 8

We report the complex dielectric function of high-quality Na3Ir3O8 single crystals determined by spectroscopic ellipsometry in the spectral range from 15 meV to 2 eV. The far-infrared phonon spectra exhibit highly asymmetric line shapes characteristic of Fano resonances. With decreasing temperature, we observe a sharp increase of the infrared intensity of the Fano-shaped phonon modes accompanied by concomitant changes in the low-energy electronic background, formed by electronic transitions between Ir 5d t2g bands of a mostly Jeff=1/2 character. The role of the complex hyperkagome lattice structure and strong spin-orbit coupling is considered.