Phonon-assisted optical excitation in the narrow bandgap Mott insulator Sr3 Ir2 O7

We examined the temperature (T) evolution of the optical conductivity spectra of Sr3Ir2O7 over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of ≤0.1 eV. The low-energy features of the optical d-d excitation (ω < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge x-ray-absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr3Ir2O7.