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 (ℏω< 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 involve, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr3Ir2O7.
@article{arxiv.1401.1325,
title = {Phonon-assisted optical excitation in the narrow bandgap Mott insulator Sr3Ir2O7},
author = {H. J. Park and C. H. Sohn and D. W. Jeong and G. Cao and K. W. Kim and S. J. Moon and Hosub Jin and Deok-Yong Cho and T. W. Noh},
journal= {arXiv preprint arXiv:1401.1325},
year = {2015}
}