Orbital-Selective Spin-Orbit Mott Insulator in Fractional Valence Iridate La$_3$Ir$_3$O$_{11}$

Abstract

The combination of strong spin-orbit coupling and Coulomb interactions makes the $5d$ iridates a unique platform for realizing novel correlated electronic states. Here, utilizing infrared spectroscopy, we demonstrate that a robust Mott insulating state persists in the $1/3$-hole self-doped system La$_3$Ir$_3$O$_{11}$, evidenced by the collapse of the Drude response and the emergence of sharp excitations across the Mott gap. Our theoretical calculations reveal that the insulating behavior arises from the cooperative interplay of structural distortions, spin-orbit coupling, and Coulomb interactions. Specifically, octahedral distortion and Ir-Ir dimerization split the $t_{2g}$ orbitals, driving the $J_{\mathrm{eff}} = 1/2$ bands toward half-filling while keeping the $J_{\mathrm{eff}} = 3/2$ bands away from it. Consequently, electron correlations induce an orbital-selective Mott transition in the $J_{\mathrm{eff}} = 1/2$ bands, whereas a band-insulating gap develops in the $J_{\mathrm{eff}} = 3/2$ bands, thereby stabilizing the unconventional insulating state in La$_3$Ir$_3$O$_{11}$. These findings provide new insights into the design and understanding of the insulating ground state of spin-orbit-coupled iridates.

Figures (3)

• Figure 1: (color online) (a) Temperature-dependent reflectivity R($\omega$) of La$_3$Ir$_3$O$_{11}$ up to 0.3 eV. The dashed line represents the corresponding low-energy extrapolation. Inset: Spectrum up to 3 eV at 300 K. (b) Temperature-dependent optical conductivity up to 0.3 eV. Symbols on the $y$ axis denote dc conductivity values at corresponding temperatures from transport data. (c) Optical conductivity up to 1.2 eV to show the spectral changes at high frequencies. Inset: the energy dependence of spectral weight at several selected temperatures. (d) Comparison of $\sigma_1(\omega)$ between La$_3$Ir$_3$O$_{11}$ and the $5d^5$ iridates Sr$_2$IrO$_{4}$Xu2020, Sr$_3$Ir$_2$O$_{7}$Seo2017, and SrIrO$_{3}$Fujioka2018 at 10 K. (e--h) Diagrams of density of states for different materials.
• Figure 2: (color online) (a) Drude-Lorentz fits applied to $\sigma_1(\omega)$ spectra. (b--d), Temperature dependence of spectral weight for different components. $\Delta S(T) = S(T) - S(300~\mathrm{K})$ represents the difference in spectral weight relative to the value at 300 K.
• Figure 3: (color online) Tight-binding band structure and orbital-resolved density of states (DOS) for La$_3$Ir$_{3}$O$_{11}$ with (a,b) dimerization only, (c,d) dimerization and octahedral distortion, and (e,f) dimerization, distortion, and spin-orbit coupling (SOC). (g) Crystal structure of La$_3$Ir$_{3}$O$_{11}$ and the Ir$_2$O$_{10}$ dimer structure. (h) Orbital-energy-level diagram for the IrO$_6$ octahedra under various conditions. (i) DOS schematics within the $J_{\mathrm{eff}}$ framework for La$_3$Ir$_{3}$O$_{11}$ under the corresponding conditions. (j) Comparison between the experimental and calculated optical conductivity $\sigma_1(\omega)$.