Charge transfer energy in iridates: a hard x-ray photoelectron spectroscopy study

TL;DR

This study uses hard x-ray photoelectron spectroscopy and tight binding calculations to analyze the electronic structure of iridates, revealing their highly covalent nature and implications for magnetic interactions and potential spin-liquid states.

Contribution

It provides the first detailed experimental and theoretical analysis of charge transfer energies in double perovskite iridates, highlighting their covalent character and impact on magnetic properties.

Findings

Iridates exhibit nearly zero O 2p to Ir 5d charge transfer energy.

Double perovskite iridates are highly covalent systems.

Long-range magnetic interactions hinder Kitaev physics realization.

Abstract

We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir$^{4+}$ or Ir$^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir $5d$, O $2p$ and O $2s$ orbitals with parameters based on the downfolding of the density-functional band structure results. We found that regardless of the A and B cations, the A$_2$BIrO$_6$ iridates have essentially zero O $2p$ to Ir $5d$ charge transfer energies. Hence, double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long-ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.