Phonon-assisted optical excitation in the narrow bandgap Mott insulatorSr3Ir2O7

We examined the temperature ($T$) evolution of the optical conductivity spectra of ${Sr}_{3}$${Ir}_{2}$${O}_{7}$ over a wide range of 10--400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\ensuremath{\le}$0.1 eV. The low-energy features of the optical d-d excitation ($\ensuremath{\hbar}\ensuremath{\omega}$ 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 ${Sr}_{3}$${Ir}_{2}$${O}_{7}$.