Phonon-assisted optical excitation in the narrow bandgap Mott insulator ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$

We examined the temperature ($T$) evolution of the optical conductivity spectra of ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$ over a wide range of 10–400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\le$0.1 eV. The low-energy features of the optical d-d excitation ($\hslash \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 ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$.

Figure 1

(a) Electronic structure of ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$ (LDA + U) taken from Ref. [2]. The direct transitions α and β are indicated exemplarily. The indirect bandgap ($E_g$) is estimated to be 85 meV. (b) The experimental ${\sigma }_1$(ω) of ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$ at various $T$. The low-energy features ($\hslash \omega$ < 0.3 eV) are enhanced with increasing $T$. Inset: a semilogarithmic plot to highlight the evolution of the low-energy features.

Figure 2

O $K$-edge XAS spectra taken with two perpendicular polarizations of σ and π at $T$ = 50 and 400 K. The measurement geometry is depicted in the inset.

Figure 3

Three major optical-phonon modes: (a) bending modes in the Ir${\mathrm{O}}_6$ octahedron at 264 and 374 cm${}^{-1}$, and (b) stretching mode for Ir-O bond. (c) The $T$ evolution of the phonon energies normalized by the lowest $T$ ($T$ = 10 K) data. Softening of the 374 cm${}^{-1}$ phonons suggests the increase of the average Ir-O bond length and the decrease of orbital hybridization due to thermal vibrational motions of the ions.

Figure 4

(a) Schematic of the optical excitations including direct photoabsorption, phonon-absorbing, or phonon-emitting indirect photoabsorptions. (b) The results of the simulation for the phonon-assisted optical absorptions based on actual $J_{\mathrm{eff}}=1/2$ LDA + $U$ band structure of ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$.