Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic State in $5d$ Transition Metal Oxides ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}$ ($n=1$, 2, and $\infty$)

We investigated the electronic structures of the $5d$ Ruddlesden-Popper series ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}$ ($n=1$, 2, and $\infty$) using optical spectroscopy and first-principles calculations. As $5d$ orbitals are spatially more extended than $3d$ or $4d$ orbitals, it has been widely accepted that correlation effects are minimal in $5d$ compounds. However, we observed a Mott insulator-metal transition with a change of bandwidth as we increased $n$. In addition, the artificially synthesized perovskite ${\mathrm{SrIrO}}_3$ showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.

Figure 1

Schematic band diagrams of $5d$ ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}$ compounds, which are well described by the effective total angular moment $J_{\mathrm{eff}}$ states due to strong spin-orbit coupling: (a) Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, (b) barely insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, and (c) correlated metal ${\mathrm{SrIrO}}_3$. $E_F$ represents the Fermi level and the arrow indicates the direction for the bandwidth $W$ increase.

Figure 2

Optical conductivity spectra $\sigma (\omega )$ of (a) ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, (b) ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, and (c) ${\mathrm{SrIrO}}_3$. In (a), $\sigma (\omega )$ of other Mott insulators, such as $3d$ ${\mathrm{LaTiO}}_3$ (Ref. 20) and $4d$ ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ (Ref. 21), are shown for comparison. Peak $\alpha$ corresponds to the optical transition from the LHB to the UHB of the $J_{\mathrm{eff},1/2}$ states. Peak $\beta$ corresponds to the transition from the $J_{\mathrm{eff},3/2}$ bands to the UHB. The insets of (c) show the x-ray pole figure (left) and diffraction pattern (right) of the perovskite ${\mathrm{SrIrO}}_3$ film. The pole figure was measured around the (011) reflection of the ${\mathrm{SrIrO}}_3$ film.

Figure 3

Results from the $\mathrm{LDA}+U$ calculations including spin-orbit coupling: Band structures of (a) ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, (b) ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, and (c) ${\mathrm{SrIrO}}_3$, and total DOS of (d) ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, (e) ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, and (f) ${\mathrm{SrIrO}}_3$. The positive and negative DOS values represent spin-up and spin-down bands, respectively. The light and dark lines represent the $J_{\mathrm{eff},1/2}$ and $J_{\mathrm{eff},3/2}$ bands, respectively.

Figure 4

(a) Results from Lorentz oscillator model analysis. The solid symbols show the positions of the $\alpha$ and $\beta$ peaks, i.e., ${\omega }_{\alpha }$ and ${\omega }_{\beta }$. The open symbols indicate the widths of the $\alpha$ and $\beta$ peaks, i.e., ${\gamma }_{\alpha }$ and ${\gamma }_{\beta }$. (b) Frequency-dependent mass enhancement $\lambda (\omega )$ of correlated metal ${\mathrm{SrIrO}}_3$. The $\lambda (\omega )$ data of ${\mathrm{SrRuO}}_3$ (our data) and ${\mathrm{Sr}}_2{\mathrm{RuO}}_3$ (Ref. 24) are included for comparison.