Magnetic Excitations in Square Lattice Iridates: Contrast between Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$

TL;DR

This study uses resonant inelastic x-ray scattering to compare magnetic and spin-orbit excitations in ultra-thin Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$, revealing significant differences due to structural and electronic factors.

Contribution

It provides the first detailed comparison of magnetic excitations in Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$, highlighting the impact of bond angles, strain, and electronic structure on their properties.

Findings

Ba$_2$IrO$_4$ exhibits larger magnon and spin-orbiton bandwidths.

Epitaxial strain significantly affects magnetic excitations.

The d-level inversion present in Sr$_2$IrO$_4$ is absent in Ba$_2$IrO$_4$.

Abstract

We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba$_2$IrO$_4$, and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr$_2$IrO$_4$. Due to the 180$^\circ$ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba$_2$IrO$_4$, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr$_2$IrO$_4$ is absent in Ba$_2$IrO$_4$, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba$_2$IrO$_4$ are substantially different from those of Sr$_2$IrO$_4$, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.