Relativistic $GW$+BSE study of the optical properties of Ruddlesden-Popper iridates

TL;DR

This study uses advanced many-body perturbation theory to analyze the optical properties of Ruddlesden-Popper iridates, revealing excitonic effects and the evolution of optical features with dimensionality, but highlighting limitations in modeling metallic states.

Contribution

It presents a comprehensive GW+BSE analysis of iridates' optical properties, including the effects of spin-orbit coupling and the limitations of simplified models.

Findings

Reproduces experimental double-peak optical spectra for insulating iridates.

Shows the redshift of optical peaks correlates with decreasing electronic correlations.

Identifies shortcomings of model dielectric function approaches for metallic iridates.

Abstract

We study the optical properties of the Ruddlesden-Popper series of iridates Sr$_{n+1}$Ir$_n$O$_{3n+1}$ ($n$=1, 2 and $\infty$) by solving the Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and screened interactions $W$ are obtained by the $GW$ approximation including spin-orbit coupling. The computed optical conductivity spectra show strong excitonic effects and reproduce very well the experimentally observed double-peak structure, in particular for the spin-orbital Mott insulators Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$. However, $GW$ does not account well for the correlated metallic state of SrIrO$_3$ owing to a much too small band renormalization, and this affects the overall quality of the optical conductivity. Our analysis describes well the progressive redshift of the main optical peaks as a function of dimensionality ($n$), which is correlated with the gradual decrease of the electronic correlation (quantified by the constrained random phase approximation) towards the metallic $n=\infty$ limit. We have also assessed the quality of a computationally cheaper BSE approach that is based on a model dielectric function and conducted on top of DFT+$U$ one-electron energies. Unfortunately, this model BSE approach does not accurately reproduce the outcome of the full $GW$+BSE method and leads to larger deviations to the measured spectra.